Merge remote-tracking branch 'remotes/kevin/tags/for-upstream' into staging

Block layer patches:

- block: Fix forbidden use of polling in drained_end
- block: Don't wait for I/O throttling while exiting QEMU
- iotests: Use read-zeroes for the null driver to be Valgrind-friendly

# gpg: Signature made Fri 19 Jul 2019 14:30:14 BST
# gpg:                using RSA key 7F09B272C88F2FD6
# gpg: Good signature from "Kevin Wolf <kwolf@redhat.com>" [full]
# Primary key fingerprint: DC3D EB15 9A9A F95D 3D74  56FE 7F09 B272 C88F 2FD6

* remotes/kevin/tags/for-upstream:
  iotests: Test quitting with job on throttled node
  vl: Drain before (block) job cancel when quitting
  iotests: Test commit with a filter on the chain
  iotests: Add @has_quit to vm.shutdown()
  block: Loop unsafely in bdrv*drained_end()
  tests: Extend commit by drained_end test
  block: Do not poll in bdrv_do_drained_end()
  tests: Lock AioContexts in test-block-iothread
  block: Make bdrv_parent_drained_[^_]*() static
  block: Add @drained_end_counter
  tests: Add job commit by drained_end test
  block: Introduce BdrvChild.parent_quiesce_counter
  iotests: Set read-zeroes on in null block driver for Valgrind

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

25 files changed, 576 insertions, 203 deletions

diff --git a/block.c b/block.c
index 29e931e..9c94f7f 100644
--- a/block.c
+++ b/block.c
@@ -911,10 +911,11 @@ static bool bdrv_child_cb_drained_poll(BdrvChild *child)
     return bdrv_drain_poll(bs, false, NULL, false);
 }
 
-static void bdrv_child_cb_drained_end(BdrvChild *child)
+static void bdrv_child_cb_drained_end(BdrvChild *child,
+                                      int *drained_end_counter)
 {
     BlockDriverState *bs = child->opaque;
-    bdrv_drained_end(bs);
+    bdrv_drained_end_no_poll(bs, drained_end_counter);
 }
 
 static void bdrv_child_cb_attach(BdrvChild *child)
@@ -2251,24 +2252,19 @@ static void bdrv_replace_child_noperm(BdrvChild *child,
         if (child->role->detach) {
             child->role->detach(child);
         }
-        if (old_bs->quiesce_counter && child->role->drained_end) {
-            int num = old_bs->quiesce_counter;
-            if (child->role->parent_is_bds) {
-                num -= bdrv_drain_all_count;
-            }
-            assert(num >= 0);
-            for (i = 0; i < num; i++) {
-                child->role->drained_end(child);
-            }
+        while (child->parent_quiesce_counter) {
+            bdrv_parent_drained_end_single(child);
         }
         QLIST_REMOVE(child, next_parent);
+    } else {
+        assert(child->parent_quiesce_counter == 0);
     }
 
     child->bs = new_bs;
 
     if (new_bs) {
         QLIST_INSERT_HEAD(&new_bs->parents, child, next_parent);
-        if (new_bs->quiesce_counter && child->role->drained_begin) {
+        if (new_bs->quiesce_counter) {
             int num = new_bs->quiesce_counter;
             if (child->role->parent_is_bds) {
                 num -= bdrv_drain_all_count;
@@ -5928,9 +5924,11 @@ static void bdrv_attach_aio_context(BlockDriverState *bs,
 void bdrv_set_aio_context_ignore(BlockDriverState *bs,
                                  AioContext *new_context, GSList **ignore)
 {
+    AioContext *old_context = bdrv_get_aio_context(bs);
+    AioContext *current_context = qemu_get_current_aio_context();
     BdrvChild *child;
 
-    if (bdrv_get_aio_context(bs) == new_context) {
+    if (old_context == new_context) {
         return;
     }
 
@@ -5954,13 +5952,31 @@ void bdrv_set_aio_context_ignore(BlockDriverState *bs,
 
     bdrv_detach_aio_context(bs);
 
-    /* This function executes in the old AioContext so acquire the new one in
-     * case it runs in a different thread.
-     */
-    aio_context_acquire(new_context);
+    /* Acquire the new context, if necessary */
+    if (current_context != new_context) {
+        aio_context_acquire(new_context);
+    }
+
     bdrv_attach_aio_context(bs, new_context);
+
+    /*
+     * If this function was recursively called from
+     * bdrv_set_aio_context_ignore(), there may be nodes in the
+     * subtree that have not yet been moved to the new AioContext.
+     * Release the old one so bdrv_drained_end() can poll them.
+     */
+    if (current_context != old_context) {
+        aio_context_release(old_context);
+    }
+
     bdrv_drained_end(bs);
-    aio_context_release(new_context);
+
+    if (current_context != old_context) {
+        aio_context_acquire(old_context);
+    }
+    if (current_context != new_context) {
+        aio_context_release(new_context);
+    }
 }
 
 static bool bdrv_parent_can_set_aio_context(BdrvChild *c, AioContext *ctx,
diff --git a/block/block-backend.c b/block/block-backend.c
index a8d160f..0056b52 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -121,7 +121,7 @@ static void blk_root_inherit_options(int *child_flags, QDict *child_options,
 }
 static void blk_root_drained_begin(BdrvChild *child);
 static bool blk_root_drained_poll(BdrvChild *child);
-static void blk_root_drained_end(BdrvChild *child);
+static void blk_root_drained_end(BdrvChild *child, int *drained_end_counter);
 
 static void blk_root_change_media(BdrvChild *child, bool load);
 static void blk_root_resize(BdrvChild *child);
@@ -1249,7 +1249,7 @@ int blk_pread_unthrottled(BlockBackend *blk, int64_t offset, uint8_t *buf,
 
     blk_root_drained_begin(blk->root);
     ret = blk_pread(blk, offset, buf, count);
-    blk_root_drained_end(blk->root);
+    blk_root_drained_end(blk->root, NULL);
     return ret;
 }
 
@@ -2236,7 +2236,7 @@ static bool blk_root_drained_poll(BdrvChild *child)
     return !!blk->in_flight;
 }
 
-static void blk_root_drained_end(BdrvChild *child)
+static void blk_root_drained_end(BdrvChild *child, int *drained_end_counter)
 {
     BlockBackend *blk = child->opaque;
     assert(blk->quiesce_counter);
diff --git a/block/io.c b/block/io.c
index 24a1875..b89e155 100644
--- a/block/io.c
+++ b/block/io.c
@@ -42,8 +42,8 @@ static void bdrv_parent_cb_resize(BlockDriverState *bs);
 static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
     int64_t offset, int bytes, BdrvRequestFlags flags);
 
-void bdrv_parent_drained_begin(BlockDriverState *bs, BdrvChild *ignore,
-                               bool ignore_bds_parents)
+static void bdrv_parent_drained_begin(BlockDriverState *bs, BdrvChild *ignore,
+                                      bool ignore_bds_parents)
 {
     BdrvChild *c, *next;
 
@@ -55,18 +55,34 @@ void bdrv_parent_drained_begin(BlockDriverState *bs, BdrvChild *ignore,
     }
 }
 
-void bdrv_parent_drained_end(BlockDriverState *bs, BdrvChild *ignore,
-                             bool ignore_bds_parents)
+static void bdrv_parent_drained_end_single_no_poll(BdrvChild *c,
+                                                   int *drained_end_counter)
 {
-    BdrvChild *c, *next;
+    assert(c->parent_quiesce_counter > 0);
+    c->parent_quiesce_counter--;
+    if (c->role->drained_end) {
+        c->role->drained_end(c, drained_end_counter);
+    }
+}
 
-    QLIST_FOREACH_SAFE(c, &bs->parents, next_parent, next) {
+void bdrv_parent_drained_end_single(BdrvChild *c)
+{
+    int drained_end_counter = 0;
+    bdrv_parent_drained_end_single_no_poll(c, &drained_end_counter);
+    BDRV_POLL_WHILE(c->bs, atomic_read(&drained_end_counter) > 0);
+}
+
+static void bdrv_parent_drained_end(BlockDriverState *bs, BdrvChild *ignore,
+                                    bool ignore_bds_parents,
+                                    int *drained_end_counter)
+{
+    BdrvChild *c;
+
+    QLIST_FOREACH(c, &bs->parents, next_parent) {
         if (c == ignore || (ignore_bds_parents && c->role->parent_is_bds)) {
             continue;
         }
-        if (c->role->drained_end) {
-            c->role->drained_end(c);
-        }
+        bdrv_parent_drained_end_single_no_poll(c, drained_end_counter);
     }
 }
 
@@ -96,6 +112,7 @@ static bool bdrv_parent_drained_poll(BlockDriverState *bs, BdrvChild *ignore,
 
 void bdrv_parent_drained_begin_single(BdrvChild *c, bool poll)
 {
+    c->parent_quiesce_counter++;
     if (c->role->drained_begin) {
         c->role->drained_begin(c);
     }
@@ -186,6 +203,7 @@ typedef struct {
     bool poll;
     BdrvChild *parent;
     bool ignore_bds_parents;
+    int *drained_end_counter;
 } BdrvCoDrainData;
 
 static void coroutine_fn bdrv_drain_invoke_entry(void *opaque)
@@ -203,13 +221,16 @@ static void coroutine_fn bdrv_drain_invoke_entry(void *opaque)
     atomic_mb_set(&data->done, true);
     bdrv_dec_in_flight(bs);
 
-    if (data->begin) {
-        g_free(data);
+    if (!data->begin) {
+        atomic_dec(data->drained_end_counter);
     }
+
+    g_free(data);
 }
 
 /* Recursively call BlockDriver.bdrv_co_drain_begin/end callbacks */
-static void bdrv_drain_invoke(BlockDriverState *bs, bool begin)
+static void bdrv_drain_invoke(BlockDriverState *bs, bool begin,
+                              int *drained_end_counter)
 {
     BdrvCoDrainData *data;
 
@@ -222,19 +243,19 @@ static void bdrv_drain_invoke(BlockDriverState *bs, bool begin)
     *data = (BdrvCoDrainData) {
         .bs = bs,
         .done = false,
-        .begin = begin
+        .begin = begin,
+        .drained_end_counter = drained_end_counter,
     };
 
+    if (!begin) {
+        atomic_inc(drained_end_counter);
+    }
+
     /* Make sure the driver callback completes during the polling phase for
      * drain_begin. */
     bdrv_inc_in_flight(bs);
     data->co = qemu_coroutine_create(bdrv_drain_invoke_entry, data);
     aio_co_schedule(bdrv_get_aio_context(bs), data->co);
-
-    if (!begin) {
-        BDRV_POLL_WHILE(bs, !data->done);
-        g_free(data);
-    }
 }
 
 /* Returns true if BDRV_POLL_WHILE() should go into a blocking aio_poll() */
@@ -273,7 +294,8 @@ static void bdrv_do_drained_begin(BlockDriverState *bs, bool recursive,
                                   BdrvChild *parent, bool ignore_bds_parents,
                                   bool poll);
 static void bdrv_do_drained_end(BlockDriverState *bs, bool recursive,
-                                BdrvChild *parent, bool ignore_bds_parents);
+                                BdrvChild *parent, bool ignore_bds_parents,
+                                int *drained_end_counter);
 
 static void bdrv_co_drain_bh_cb(void *opaque)
 {
@@ -296,11 +318,14 @@ static void bdrv_co_drain_bh_cb(void *opaque)
         }
         bdrv_dec_in_flight(bs);
         if (data->begin) {
+            assert(!data->drained_end_counter);
             bdrv_do_drained_begin(bs, data->recursive, data->parent,
                                   data->ignore_bds_parents, data->poll);
         } else {
+            assert(!data->poll);
             bdrv_do_drained_end(bs, data->recursive, data->parent,
-                                data->ignore_bds_parents);
+                                data->ignore_bds_parents,
+                                data->drained_end_counter);
         }
         if (ctx == co_ctx) {
             aio_context_release(ctx);
@@ -318,7 +343,8 @@ static void coroutine_fn bdrv_co_yield_to_drain(BlockDriverState *bs,
                                                 bool begin, bool recursive,
                                                 BdrvChild *parent,
                                                 bool ignore_bds_parents,
-                                                bool poll)
+                                                bool poll,
+                                                int *drained_end_counter)
 {
     BdrvCoDrainData data;
 
@@ -335,7 +361,9 @@ static void coroutine_fn bdrv_co_yield_to_drain(BlockDriverState *bs,
         .parent = parent,
         .ignore_bds_parents = ignore_bds_parents,
         .poll = poll,
+        .drained_end_counter = drained_end_counter,
     };
+
     if (bs) {
         bdrv_inc_in_flight(bs);
     }
@@ -359,7 +387,7 @@ void bdrv_do_drained_begin_quiesce(BlockDriverState *bs,
     }
 
     bdrv_parent_drained_begin(bs, parent, ignore_bds_parents);
-    bdrv_drain_invoke(bs, true);
+    bdrv_drain_invoke(bs, true, NULL);
 }
 
 static void bdrv_do_drained_begin(BlockDriverState *bs, bool recursive,
@@ -370,7 +398,7 @@ static void bdrv_do_drained_begin(BlockDriverState *bs, bool recursive,
 
     if (qemu_in_coroutine()) {
         bdrv_co_yield_to_drain(bs, true, recursive, parent, ignore_bds_parents,
-                               poll);
+                               poll, NULL);
         return;
     }
 
@@ -410,22 +438,40 @@ void bdrv_subtree_drained_begin(BlockDriverState *bs)
     bdrv_do_drained_begin(bs, true, NULL, false, true);
 }
 
+/**
+ * This function does not poll, nor must any of its recursively called
+ * functions.  The *drained_end_counter pointee will be incremented
+ * once for every background operation scheduled, and decremented once
+ * the operation settles.  Therefore, the pointer must remain valid
+ * until the pointee reaches 0.  That implies that whoever sets up the
+ * pointee has to poll until it is 0.
+ *
+ * We use atomic operations to access *drained_end_counter, because
+ * (1) when called from bdrv_set_aio_context_ignore(), the subgraph of
+ *     @bs may contain nodes in different AioContexts,
+ * (2) bdrv_drain_all_end() uses the same counter for all nodes,
+ *     regardless of which AioContext they are in.
+ */
 static void bdrv_do_drained_end(BlockDriverState *bs, bool recursive,
-                                BdrvChild *parent, bool ignore_bds_parents)
+                                BdrvChild *parent, bool ignore_bds_parents,
+                                int *drained_end_counter)
 {
-    BdrvChild *child, *next;
+    BdrvChild *child;
     int old_quiesce_counter;
 
+    assert(drained_end_counter != NULL);
+
     if (qemu_in_coroutine()) {
         bdrv_co_yield_to_drain(bs, false, recursive, parent, ignore_bds_parents,
-                               false);
+                               false, drained_end_counter);
         return;
     }
     assert(bs->quiesce_counter > 0);
 
     /* Re-enable things in child-to-parent order */
-    bdrv_drain_invoke(bs, false);
-    bdrv_parent_drained_end(bs, parent, ignore_bds_parents);
+    bdrv_drain_invoke(bs, false, drained_end_counter);
+    bdrv_parent_drained_end(bs, parent, ignore_bds_parents,
+                            drained_end_counter);
 
     old_quiesce_counter = atomic_fetch_dec(&bs->quiesce_counter);
     if (old_quiesce_counter == 1) {
@@ -435,20 +481,30 @@ static void bdrv_do_drained_end(BlockDriverState *bs, bool recursive,
     if (recursive) {
         assert(!ignore_bds_parents);
         bs->recursive_quiesce_counter--;
-        QLIST_FOREACH_SAFE(child, &bs->children, next, next) {
-            bdrv_do_drained_end(child->bs, true, child, ignore_bds_parents);
+        QLIST_FOREACH(child, &bs->children, next) {
+            bdrv_do_drained_end(child->bs, true, child, ignore_bds_parents,
+                                drained_end_counter);
         }
     }
 }
 
 void bdrv_drained_end(BlockDriverState *bs)
 {
-    bdrv_do_drained_end(bs, false, NULL, false);
+    int drained_end_counter = 0;
+    bdrv_do_drained_end(bs, false, NULL, false, &drained_end_counter);
+    BDRV_POLL_WHILE(bs, atomic_read(&drained_end_counter) > 0);
+}
+
+void bdrv_drained_end_no_poll(BlockDriverState *bs, int *drained_end_counter)
+{
+    bdrv_do_drained_end(bs, false, NULL, false, drained_end_counter);
 }
 
 void bdrv_subtree_drained_end(BlockDriverState *bs)
 {
-    bdrv_do_drained_end(bs, true, NULL, false);
+    int drained_end_counter = 0;
+    bdrv_do_drained_end(bs, true, NULL, false, &drained_end_counter);
+    BDRV_POLL_WHILE(bs, atomic_read(&drained_end_counter) > 0);
 }
 
 void bdrv_apply_subtree_drain(BdrvChild *child, BlockDriverState *new_parent)
@@ -462,11 +518,15 @@ void bdrv_apply_subtree_drain(BdrvChild *child, BlockDriverState *new_parent)
 
 void bdrv_unapply_subtree_drain(BdrvChild *child, BlockDriverState *old_parent)
 {
+    int drained_end_counter = 0;
     int i;
 
     for (i = 0; i < old_parent->recursive_quiesce_counter; i++) {
-        bdrv_do_drained_end(child->bs, true, child, false);
+        bdrv_do_drained_end(child->bs, true, child, false,
+                            &drained_end_counter);
     }
+
+    BDRV_POLL_WHILE(child->bs, atomic_read(&drained_end_counter) > 0);
 }
 
 /*
@@ -535,7 +595,7 @@ void bdrv_drain_all_begin(void)
     BlockDriverState *bs = NULL;
 
     if (qemu_in_coroutine()) {
-        bdrv_co_yield_to_drain(NULL, true, false, NULL, true, true);
+        bdrv_co_yield_to_drain(NULL, true, false, NULL, true, true, NULL);
         return;
     }
 
@@ -566,15 +626,19 @@ void bdrv_drain_all_begin(void)
 void bdrv_drain_all_end(void)
 {
     BlockDriverState *bs = NULL;
+    int drained_end_counter = 0;
 
     while ((bs = bdrv_next_all_states(bs))) {
         AioContext *aio_context = bdrv_get_aio_context(bs);
 
         aio_context_acquire(aio_context);
-        bdrv_do_drained_end(bs, false, NULL, true);
+        bdrv_do_drained_end(bs, false, NULL, true, &drained_end_counter);
         aio_context_release(aio_context);
     }
 
+    assert(qemu_get_current_aio_context() == qemu_get_aio_context());
+    AIO_WAIT_WHILE(NULL, atomic_read(&drained_end_counter) > 0);
+
     assert(bdrv_drain_all_count > 0);
     bdrv_drain_all_count--;
 }
diff --git a/blockjob.c b/blockjob.c
index 458ae76..20b7f55 100644
--- a/blockjob.c
+++ b/blockjob.c
@@ -135,7 +135,7 @@ static bool child_job_drained_poll(BdrvChild *c)
     }
 }
 
-static void child_job_drained_end(BdrvChild *c)
+static void child_job_drained_end(BdrvChild *c, int *drained_end_counter)
 {
     BlockJob *job = c->opaque;
     job_resume(&job->job);
diff --git a/include/block/block.h b/include/block/block.h
index 734c9d2..60f0047 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -601,15 +601,6 @@ void bdrv_io_plug(BlockDriverState *bs);
 void bdrv_io_unplug(BlockDriverState *bs);
 
 /**
- * bdrv_parent_drained_begin:
- *
- * Begin a quiesced section of all users of @bs. This is part of
- * bdrv_drained_begin.
- */
-void bdrv_parent_drained_begin(BlockDriverState *bs, BdrvChild *ignore,
-                               bool ignore_bds_parents);
-
-/**
  * bdrv_parent_drained_begin_single:
  *
  * Begin a quiesced section for the parent of @c. If @poll is true, wait for
@@ -618,13 +609,14 @@ void bdrv_parent_drained_begin(BlockDriverState *bs, BdrvChild *ignore,
 void bdrv_parent_drained_begin_single(BdrvChild *c, bool poll);
 
 /**
- * bdrv_parent_drained_end:
+ * bdrv_parent_drained_end_single:
+ *
+ * End a quiesced section for the parent of @c.
  *
- * End a quiesced section of all users of @bs. This is part of
- * bdrv_drained_end.
+ * This polls @bs's AioContext until all scheduled sub-drained_ends
+ * have settled, which may result in graph changes.
  */
-void bdrv_parent_drained_end(BlockDriverState *bs, BdrvChild *ignore,
-                             bool ignore_bds_parents);
+void bdrv_parent_drained_end_single(BdrvChild *c);
 
 /**
  * bdrv_drain_poll:
@@ -672,10 +664,32 @@ void bdrv_subtree_drained_begin(BlockDriverState *bs);
  * bdrv_drained_end:
  *
  * End a quiescent section started by bdrv_drained_begin().
+ *
+ * This polls @bs's AioContext until all scheduled sub-drained_ends
+ * have settled.  On one hand, that may result in graph changes.  On
+ * the other, this requires that all involved nodes (@bs and all of
+ * its parents) are in the same AioContext, and that the caller has
+ * acquired it.
+ * If there are any nodes that are in different contexts from @bs,
+ * these contexts must not be acquired.
  */
 void bdrv_drained_end(BlockDriverState *bs);
 
 /**
+ * bdrv_drained_end_no_poll:
+ *
+ * Same as bdrv_drained_end(), but do not poll for the subgraph to
+ * actually become unquiesced.  Therefore, no graph changes will occur
+ * with this function.
+ *
+ * *drained_end_counter is incremented for every background operation
+ * that is scheduled, and will be decremented for every operation once
+ * it settles.  The caller must poll until it reaches 0.  The counter
+ * should be accessed using atomic operations only.
+ */
+void bdrv_drained_end_no_poll(BlockDriverState *bs, int *drained_end_counter);
+
+/**
  * End a quiescent section started by bdrv_subtree_drained_begin().
  */
 void bdrv_subtree_drained_end(BlockDriverState *bs);
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 5090253..3aa1e83 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -664,11 +664,15 @@ struct BdrvChildRole {
      * These functions must not change the graph (and therefore also must not
      * call aio_poll(), which could change the graph indirectly).
      *
+     * If drained_end() schedules background operations, it must atomically
+     * increment *drained_end_counter for each such operation and atomically
+     * decrement it once the operation has settled.
+     *
      * Note that this can be nested. If drained_begin() was called twice, new
      * I/O is allowed only after drained_end() was called twice, too.
      */
     void (*drained_begin)(BdrvChild *child);
-    void (*drained_end)(BdrvChild *child);
+    void (*drained_end)(BdrvChild *child, int *drained_end_counter);
 
     /*
      * Returns whether the parent has pending requests for the child. This
@@ -729,6 +733,15 @@ struct BdrvChild {
      */
     bool frozen;
 
+    /*
+     * How many times the parent of this child has been drained
+     * (through role->drained_*).
+     * Usually, this is equal to bs->quiesce_counter (potentially
+     * reduced by bdrv_drain_all_count).  It may differ while the
+     * child is entering or leaving a drained section.
+     */
+    int parent_quiesce_counter;
+
     QLIST_ENTRY(BdrvChild) next;
     QLIST_ENTRY(BdrvChild) next_parent;
 };
diff --git a/python/qemu/machine.py b/python/qemu/machine.py
index 49445e6..128a3d1 100644
--- a/python/qemu/machine.py
+++ b/python/qemu/machine.py
@@ -329,13 +329,14 @@ class QEMUMachine(object):
         self._load_io_log()
         self._post_shutdown()
 
-    def shutdown(self):
+    def shutdown(self, has_quit=False):
         """
         Terminate the VM and clean up
         """
         if self.is_running():
             try:
-                self._qmp.cmd('quit')
+                if not has_quit:
+                    self._qmp.cmd('quit')
                 self._qmp.close()
             except:
                 self._popen.kill()
diff --git a/tests/qemu-iotests/040 b/tests/qemu-iotests/040
index b81133a..aa0b184 100755
--- a/tests/qemu-iotests/040
+++ b/tests/qemu-iotests/040
@@ -92,9 +92,10 @@ class TestSingleDrive(ImageCommitTestCase):
 
         self.vm.add_device("scsi-hd,id=scsi0,drive=drive0")
         self.vm.launch()
+        self.has_quit = False
 
     def tearDown(self):
-        self.vm.shutdown()
+        self.vm.shutdown(has_quit=self.has_quit)
         os.remove(test_img)
         os.remove(mid_img)
         os.remove(backing_img)
@@ -109,6 +110,43 @@ class TestSingleDrive(ImageCommitTestCase):
         self.assertEqual(-1, qemu_io('-f', 'raw', '-c', 'read -P 0xab 0 524288', backing_img).find("verification failed"))
         self.assertEqual(-1, qemu_io('-f', 'raw', '-c', 'read -P 0xef 524288 524288', backing_img).find("verification failed"))
 
+    def test_commit_with_filter_and_quit(self):
+        result = self.vm.qmp('object-add', qom_type='throttle-group', id='tg')
+        self.assert_qmp(result, 'return', {})
+
+        # Add a filter outside of the backing chain
+        result = self.vm.qmp('blockdev-add', driver='throttle', node_name='filter', throttle_group='tg', file='mid')
+        self.assert_qmp(result, 'return', {})
+
+        result = self.vm.qmp('block-commit', device='drive0')
+        self.assert_qmp(result, 'return', {})
+
+        # Quit immediately, thus forcing a simultaneous cancel of the
+        # block job and a bdrv_drain_all()
+        result = self.vm.qmp('quit')
+        self.assert_qmp(result, 'return', {})
+
+        self.has_quit = True
+
+    # Same as above, but this time we add the filter after starting the job
+    def test_commit_plus_filter_and_quit(self):
+        result = self.vm.qmp('object-add', qom_type='throttle-group', id='tg')
+        self.assert_qmp(result, 'return', {})
+
+        result = self.vm.qmp('block-commit', device='drive0')
+        self.assert_qmp(result, 'return', {})
+
+        # Add a filter outside of the backing chain
+        result = self.vm.qmp('blockdev-add', driver='throttle', node_name='filter', throttle_group='tg', file='mid')
+        self.assert_qmp(result, 'return', {})
+
+        # Quit immediately, thus forcing a simultaneous cancel of the
+        # block job and a bdrv_drain_all()
+        result = self.vm.qmp('quit')
+        self.assert_qmp(result, 'return', {})
+
+        self.has_quit = True
+
     def test_device_not_found(self):
         result = self.vm.qmp('block-commit', device='nonexistent', top='%s' % mid_img)
         self.assert_qmp(result, 'error/class', 'DeviceNotFound')
diff --git a/tests/qemu-iotests/040.out b/tests/qemu-iotests/040.out
index 802ffaa..220a5fa 100644
--- a/tests/qemu-iotests/040.out
+++ b/tests/qemu-iotests/040.out
@@ -1,5 +1,5 @@
-...........................................
+...............................................
 ----------------------------------------------------------------------
-Ran 43 tests
+Ran 47 tests
 
 OK
diff --git a/tests/qemu-iotests/051 b/tests/qemu-iotests/051
index 200660f..ce942a5 100755
--- a/tests/qemu-iotests/051
+++ b/tests/qemu-iotests/051
@@ -251,11 +251,11 @@ echo
 # Cannot use the test image because cache=none might not work on the host FS
 # Use cdrom so that we won't get errors about missing media
 
-run_qemu -drive driver=null-co,cache=none
-run_qemu -drive driver=null-co,cache=directsync
-run_qemu -drive driver=null-co,cache=writeback
-run_qemu -drive driver=null-co,cache=writethrough
-run_qemu -drive driver=null-co,cache=unsafe
+run_qemu -drive driver=null-co,read-zeroes=on,cache=none
+run_qemu -drive driver=null-co,read-zeroes=on,cache=directsync
+run_qemu -drive driver=null-co,read-zeroes=on,cache=writeback
+run_qemu -drive driver=null-co,read-zeroes=on,cache=writethrough
+run_qemu -drive driver=null-co,read-zeroes=on,cache=unsafe
 run_qemu -drive driver=null-co,cache=invalid_value
 
 # Can't test direct=on here because O_DIRECT might not be supported on this FS
diff --git a/tests/qemu-iotests/051.pc.out b/tests/qemu-iotests/051.pc.out
index 2d811c1..000557c 100644
--- a/tests/qemu-iotests/051.pc.out
+++ b/tests/qemu-iotests/051.pc.out
@@ -245,23 +245,23 @@ QEMU X.Y.Z monitor - type 'help' for more information
 
 === Cache modes ===
 
-Testing: -drive driver=null-co,cache=none
+Testing: -drive driver=null-co,read-zeroes=on,cache=none
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) quit
 
-Testing: -drive driver=null-co,cache=directsync
+Testing: -drive driver=null-co,read-zeroes=on,cache=directsync
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) quit
 
-Testing: -drive driver=null-co,cache=writeback
+Testing: -drive driver=null-co,read-zeroes=on,cache=writeback
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) quit
 
-Testing: -drive driver=null-co,cache=writethrough
+Testing: -drive driver=null-co,read-zeroes=on,cache=writethrough
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) quit
 
-Testing: -drive driver=null-co,cache=unsafe
+Testing: -drive driver=null-co,read-zeroes=on,cache=unsafe
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) quit
 
diff --git a/tests/qemu-iotests/093 b/tests/qemu-iotests/093
index d88fbc1..4b2cac1 100755
--- a/tests/qemu-iotests/093
+++ b/tests/qemu-iotests/093
@@ -38,7 +38,7 @@ class ThrottleTestCase(iotests.QMPTestCase):
     def setUp(self):
         self.vm = iotests.VM()
         for i in range(0, self.max_drives):
-            self.vm.add_drive(self.test_img)
+            self.vm.add_drive(self.test_img, "file.read-zeroes=on")
         self.vm.launch()
 
     def tearDown(self):
@@ -273,7 +273,8 @@ class ThrottleTestGroupNames(iotests.QMPTestCase):
     def setUp(self):
         self.vm = iotests.VM()
         for i in range(0, self.max_drives):
-            self.vm.add_drive(self.test_img, "throttling.iops-total=100")
+            self.vm.add_drive(self.test_img,
+                              "throttling.iops-total=100,file.read-zeroes=on")
         self.vm.launch()
 
     def tearDown(self):
@@ -378,10 +379,10 @@ class ThrottleTestRemovableMedia(iotests.QMPTestCase):
     def test_removable_media(self):
         # Add a couple of dummy nodes named cd0 and cd1
         result = self.vm.qmp("blockdev-add", driver="null-aio",
-                             node_name="cd0")
+                             read_zeroes=True, node_name="cd0")
         self.assert_qmp(result, 'return', {})
         result = self.vm.qmp("blockdev-add", driver="null-aio",
-                             node_name="cd1")
+                             read_zeroes=True, node_name="cd1")
         self.assert_qmp(result, 'return', {})
 
         # Attach a CD drive with cd0 inserted
diff --git a/tests/qemu-iotests/136 b/tests/qemu-iotests/136
index af7ffa4..a46a7b7 100755
--- a/tests/qemu-iotests/136
+++ b/tests/qemu-iotests/136
@@ -74,6 +74,7 @@ sector = "%d"
                           (self.account_invalid and "on" or "off"))
         drive_args.append("stats-account-failed=%s" %
                           (self.account_failed and "on" or "off"))
+        drive_args.append("file.image.read-zeroes=on")
         self.create_blkdebug_file()
         self.vm = iotests.VM().add_drive('blkdebug:%s:%s' %
                                          (blkdebug_file, self.test_img),
diff --git a/tests/qemu-iotests/186 b/tests/qemu-iotests/186
index 7e7d45b..5f6b18c 100755
--- a/tests/qemu-iotests/186
+++ b/tests/qemu-iotests/186
@@ -86,8 +86,8 @@ echo "=== -blockdev/-device=<node-name> ==="
 echo
 
 for dev in $fixed $removable; do
-    check_info_block -blockdev driver=null-co,node-name=null -device $dev,drive=null
-    check_info_block -blockdev driver=null-co,node-name=null -device $dev,drive=null,id=qdev_id
+    check_info_block -blockdev driver=null-co,read-zeroes=on,node-name=null -device $dev,drive=null
+    check_info_block -blockdev driver=null-co,read-zeroes=on,node-name=null -device $dev,drive=null,id=qdev_id
 done
 
 echo
@@ -97,7 +97,7 @@ echo
 # This creates two BlockBackends that will show up in 'info block'!
 # A monitor-owned one from -drive, and anonymous one from -device
 for dev in $fixed $removable; do
-    check_info_block -drive if=none,driver=null-co,node-name=null -device $dev,drive=null,id=qdev_id
+    check_info_block -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device $dev,drive=null,id=qdev_id
 done
 
 echo
@@ -105,8 +105,8 @@ echo "=== -drive if=none/-device=<bb-name> (with medium) ==="
 echo
 
 for dev in $fixed $removable; do
-    check_info_block -drive if=none,driver=null-co,node-name=null -device $dev,drive=none0
-    check_info_block -drive if=none,driver=null-co,node-name=null -device $dev,drive=none0,id=qdev_id
+    check_info_block -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device $dev,drive=none0
+    check_info_block -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device $dev,drive=none0,id=qdev_id
 done
 
 echo
@@ -125,15 +125,15 @@ echo "=== -drive if=... ==="
 echo
 
 check_info_block -drive if=floppy
-check_info_block -drive if=floppy,driver=null-co
+check_info_block -drive if=floppy,driver=null-co,read-zeroes=on
 
-check_info_block -drive if=ide,driver=null-co
+check_info_block -drive if=ide,driver=null-co,read-zeroes=on
 check_info_block -drive if=ide,media=cdrom
-check_info_block -drive if=ide,driver=null-co,media=cdrom
+check_info_block -drive if=ide,driver=null-co,read-zeroes=on,media=cdrom
 
-check_info_block -drive if=virtio,driver=null-co
+check_info_block -drive if=virtio,driver=null-co,read-zeroes=on
 
-check_info_block -drive if=pflash,driver=null-co,size=1M
+check_info_block -drive if=pflash,driver=null-co,read-zeroes=on,size=1M
 
 # success, all done
 echo "*** done"
diff --git a/tests/qemu-iotests/186.out b/tests/qemu-iotests/186.out
index 716b01a..5b35040 100644
--- a/tests/qemu-iotests/186.out
+++ b/tests/qemu-iotests/186.out
@@ -54,103 +54,103 @@ qdev_id: [not inserted]
 
 === -blockdev/-device=<node-name> ===
 
-Testing: -blockdev driver=null-co,node-name=null -device ide-hd,drive=null
+Testing: -blockdev driver=null-co,read-zeroes=on,node-name=null -device ide-hd,drive=null
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-null: null-co:// (null-co)
+null: json:{"read-zeroes": true, "driver": "null-co"} (null-co)
     Attached to:      PATH
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -blockdev driver=null-co,node-name=null -device ide-hd,drive=null,id=qdev_id
+Testing: -blockdev driver=null-co,read-zeroes=on,node-name=null -device ide-hd,drive=null,id=qdev_id
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-null: null-co:// (null-co)
+null: json:{"read-zeroes": true, "driver": "null-co"} (null-co)
     Attached to:      qdev_id
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -blockdev driver=null-co,node-name=null -device scsi-hd,drive=null
+Testing: -blockdev driver=null-co,read-zeroes=on,node-name=null -device scsi-hd,drive=null
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-null: null-co:// (null-co)
+null: json:{"read-zeroes": true, "driver": "null-co"} (null-co)
     Attached to:      PATH
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -blockdev driver=null-co,node-name=null -device scsi-hd,drive=null,id=qdev_id
+Testing: -blockdev driver=null-co,read-zeroes=on,node-name=null -device scsi-hd,drive=null,id=qdev_id
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-null: null-co:// (null-co)
+null: json:{"read-zeroes": true, "driver": "null-co"} (null-co)
     Attached to:      qdev_id
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -blockdev driver=null-co,node-name=null -device virtio-blk-pci,drive=null
+Testing: -blockdev driver=null-co,read-zeroes=on,node-name=null -device virtio-blk-pci,drive=null
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-null: null-co:// (null-co)
+null: json:{"read-zeroes": true, "driver": "null-co"} (null-co)
     Attached to:      PATH
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -blockdev driver=null-co,node-name=null -device virtio-blk-pci,drive=null,id=qdev_id
+Testing: -blockdev driver=null-co,read-zeroes=on,node-name=null -device virtio-blk-pci,drive=null,id=qdev_id
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-null: null-co:// (null-co)
+null: json:{"read-zeroes": true, "driver": "null-co"} (null-co)
     Attached to:      PATH
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -blockdev driver=null-co,node-name=null -device floppy,drive=null
+Testing: -blockdev driver=null-co,read-zeroes=on,node-name=null -device floppy,drive=null
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-null: null-co:// (null-co)
+null: json:{"read-zeroes": true, "driver": "null-co"} (null-co)
     Attached to:      PATH
     Removable device: not locked, tray closed
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -blockdev driver=null-co,node-name=null -device floppy,drive=null,id=qdev_id
+Testing: -blockdev driver=null-co,read-zeroes=on,node-name=null -device floppy,drive=null,id=qdev_id
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-null: null-co:// (null-co)
+null: json:{"read-zeroes": true, "driver": "null-co"} (null-co)
     Attached to:      qdev_id
     Removable device: not locked, tray closed
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -blockdev driver=null-co,node-name=null -device ide-cd,drive=null
+Testing: -blockdev driver=null-co,read-zeroes=on,node-name=null -device ide-cd,drive=null
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-null: null-co:// (null-co)
+null: json:{"read-zeroes": true, "driver": "null-co"} (null-co)
     Attached to:      PATH
     Removable device: not locked, tray closed
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -blockdev driver=null-co,node-name=null -device ide-cd,drive=null,id=qdev_id
+Testing: -blockdev driver=null-co,read-zeroes=on,node-name=null -device ide-cd,drive=null,id=qdev_id
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-null: null-co:// (null-co)
+null: json:{"read-zeroes": true, "driver": "null-co"} (null-co)
     Attached to:      qdev_id
     Removable device: not locked, tray closed
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -blockdev driver=null-co,node-name=null -device scsi-cd,drive=null
+Testing: -blockdev driver=null-co,read-zeroes=on,node-name=null -device scsi-cd,drive=null
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-null: null-co:// (null-co)
+null: json:{"read-zeroes": true, "driver": "null-co"} (null-co)
     Attached to:      PATH
     Removable device: not locked, tray closed
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -blockdev driver=null-co,node-name=null -device scsi-cd,drive=null,id=qdev_id
+Testing: -blockdev driver=null-co,read-zeroes=on,node-name=null -device scsi-cd,drive=null,id=qdev_id
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-null: null-co:// (null-co)
+null: json:{"read-zeroes": true, "driver": "null-co"} (null-co)
     Attached to:      qdev_id
     Removable device: not locked, tray closed
     Cache mode:       writeback
@@ -159,76 +159,76 @@ null: null-co:// (null-co)
 
 === -drive if=none/-device=<node-name> ===
 
-Testing: -drive if=none,driver=null-co,node-name=null -device ide-hd,drive=null,id=qdev_id
+Testing: -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device ide-hd,drive=null,id=qdev_id
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-none0 (null): null-co:// (null-co)
+none0 (null): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Removable device: not locked, tray closed
     Cache mode:       writeback
 
-null: null-co:// (null-co)
+null: json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      qdev_id
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -drive if=none,driver=null-co,node-name=null -device scsi-hd,drive=null,id=qdev_id
+Testing: -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device scsi-hd,drive=null,id=qdev_id
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-none0 (null): null-co:// (null-co)
+none0 (null): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Removable device: not locked, tray closed
     Cache mode:       writeback
 
-null: null-co:// (null-co)
+null: json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      qdev_id
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -drive if=none,driver=null-co,node-name=null -device virtio-blk-pci,drive=null,id=qdev_id
+Testing: -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device virtio-blk-pci,drive=null,id=qdev_id
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-none0 (null): null-co:// (null-co)
+none0 (null): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Removable device: not locked, tray closed
     Cache mode:       writeback
 
-null: null-co:// (null-co)
+null: json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      PATH
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -drive if=none,driver=null-co,node-name=null -device floppy,drive=null,id=qdev_id
+Testing: -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device floppy,drive=null,id=qdev_id
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-none0 (null): null-co:// (null-co)
+none0 (null): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Removable device: not locked, tray closed
     Cache mode:       writeback
 
-null: null-co:// (null-co)
+null: json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      qdev_id
     Removable device: not locked, tray closed
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -drive if=none,driver=null-co,node-name=null -device ide-cd,drive=null,id=qdev_id
+Testing: -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device ide-cd,drive=null,id=qdev_id
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-none0 (null): null-co:// (null-co)
+none0 (null): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Removable device: not locked, tray closed
     Cache mode:       writeback
 
-null: null-co:// (null-co)
+null: json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      qdev_id
     Removable device: not locked, tray closed
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -drive if=none,driver=null-co,node-name=null -device scsi-cd,drive=null,id=qdev_id
+Testing: -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device scsi-cd,drive=null,id=qdev_id
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-none0 (null): null-co:// (null-co)
+none0 (null): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Removable device: not locked, tray closed
     Cache mode:       writeback
 
-null: null-co:// (null-co)
+null: json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      qdev_id
     Removable device: not locked, tray closed
     Cache mode:       writeback
@@ -237,103 +237,103 @@ null: null-co:// (null-co)
 
 === -drive if=none/-device=<bb-name> (with medium) ===
 
-Testing: -drive if=none,driver=null-co,node-name=null -device ide-hd,drive=none0
+Testing: -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device ide-hd,drive=none0
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-none0 (null): null-co:// (null-co)
+none0 (null): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      PATH
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -drive if=none,driver=null-co,node-name=null -device ide-hd,drive=none0,id=qdev_id
+Testing: -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device ide-hd,drive=none0,id=qdev_id
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-none0 (null): null-co:// (null-co)
+none0 (null): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      qdev_id
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -drive if=none,driver=null-co,node-name=null -device scsi-hd,drive=none0
+Testing: -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device scsi-hd,drive=none0
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-none0 (null): null-co:// (null-co)
+none0 (null): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      PATH
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -drive if=none,driver=null-co,node-name=null -device scsi-hd,drive=none0,id=qdev_id
+Testing: -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device scsi-hd,drive=none0,id=qdev_id
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-none0 (null): null-co:// (null-co)
+none0 (null): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      qdev_id
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -drive if=none,driver=null-co,node-name=null -device virtio-blk-pci,drive=none0
+Testing: -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device virtio-blk-pci,drive=none0
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-none0 (null): null-co:// (null-co)
+none0 (null): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      PATH
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -drive if=none,driver=null-co,node-name=null -device virtio-blk-pci,drive=none0,id=qdev_id
+Testing: -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device virtio-blk-pci,drive=none0,id=qdev_id
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-none0 (null): null-co:// (null-co)
+none0 (null): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      PATH
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -drive if=none,driver=null-co,node-name=null -device floppy,drive=none0
+Testing: -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device floppy,drive=none0
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-none0 (null): null-co:// (null-co)
+none0 (null): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      PATH
     Removable device: not locked, tray closed
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -drive if=none,driver=null-co,node-name=null -device floppy,drive=none0,id=qdev_id
+Testing: -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device floppy,drive=none0,id=qdev_id
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-none0 (null): null-co:// (null-co)
+none0 (null): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      qdev_id
     Removable device: not locked, tray closed
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -drive if=none,driver=null-co,node-name=null -device ide-cd,drive=none0
+Testing: -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device ide-cd,drive=none0
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-none0 (null): null-co:// (null-co)
+none0 (null): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      PATH
     Removable device: not locked, tray closed
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -drive if=none,driver=null-co,node-name=null -device ide-cd,drive=none0,id=qdev_id
+Testing: -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device ide-cd,drive=none0,id=qdev_id
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-none0 (null): null-co:// (null-co)
+none0 (null): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      qdev_id
     Removable device: not locked, tray closed
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -drive if=none,driver=null-co,node-name=null -device scsi-cd,drive=none0
+Testing: -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device scsi-cd,drive=none0
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-none0 (null): null-co:// (null-co)
+none0 (null): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      PATH
     Removable device: not locked, tray closed
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -drive if=none,driver=null-co,node-name=null -device scsi-cd,drive=none0,id=qdev_id
+Testing: -drive if=none,driver=null-co,read-zeroes=on,node-name=null -device scsi-cd,drive=none0,id=qdev_id
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-none0 (null): null-co:// (null-co)
+none0 (null): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      qdev_id
     Removable device: not locked, tray closed
     Cache mode:       writeback
@@ -408,19 +408,19 @@ floppy0: [not inserted]
     Removable device: not locked, tray closed
 (qemu) quit
 
-Testing: -drive if=floppy,driver=null-co
+Testing: -drive if=floppy,driver=null-co,read-zeroes=on
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-floppy0 (NODE_NAME): null-co:// (null-co)
+floppy0 (NODE_NAME): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      PATH
     Removable device: not locked, tray closed
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -drive if=ide,driver=null-co
+Testing: -drive if=ide,driver=null-co,read-zeroes=on
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-ide0-hd0 (NODE_NAME): null-co:// (null-co)
+ide0-hd0 (NODE_NAME): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      PATH
     Cache mode:       writeback
 (qemu) quit
@@ -433,27 +433,27 @@ ide0-cd0: [not inserted]
     Removable device: not locked, tray closed
 (qemu) quit
 
-Testing: -drive if=ide,driver=null-co,media=cdrom
+Testing: -drive if=ide,driver=null-co,read-zeroes=on,media=cdrom
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-ide0-cd0 (NODE_NAME): null-co:// (null-co, read-only)
+ide0-cd0 (NODE_NAME): json:{"read-zeroes": "on", "driver": "null-co"} (null-co, read-only)
     Attached to:      PATH
     Removable device: not locked, tray closed
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -drive if=virtio,driver=null-co
+Testing: -drive if=virtio,driver=null-co,read-zeroes=on
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-virtio0 (NODE_NAME): null-co:// (null-co)
+virtio0 (NODE_NAME): json:{"read-zeroes": "on", "driver": "null-co"} (null-co)
     Attached to:      PATH
     Cache mode:       writeback
 (qemu) quit
 
-Testing: -drive if=pflash,driver=null-co,size=1M
+Testing: -drive if=pflash,driver=null-co,read-zeroes=on,size=1M
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) info block
-pflash0 (NODE_NAME): json:{"driver": "null-co", "size": "1M"} (null-co)
+pflash0 (NODE_NAME): json:{"read-zeroes": "on", "driver": "null-co", "size": "1M"} (null-co)
     Attached to:      PATH
     Cache mode:       writeback
 (qemu) quit
diff --git a/tests/qemu-iotests/218 b/tests/qemu-iotests/218
index 92c331b..2554d84 100755
--- a/tests/qemu-iotests/218
+++ b/tests/qemu-iotests/218
@@ -27,9 +27,9 @@
 # Creator/Owner: Max Reitz <mreitz@redhat.com>
 
 import iotests
-from iotests import log
+from iotests import log, qemu_img, qemu_io_silent
 
-iotests.verify_platform(['linux'])
+iotests.verify_image_format(supported_fmts=['qcow2', 'raw'])
 
 
 # Launches the VM, adds two null-co nodes (source and target), and
@@ -136,3 +136,54 @@ with iotests.VM() as vm:
 
     log(vm.event_wait('BLOCK_JOB_CANCELLED'),
         filters=[iotests.filter_qmp_event])
+
+log('')
+log('=== Cancel mirror job from throttled node by quitting ===')
+log('')
+
+with iotests.VM() as vm, \
+     iotests.FilePath('src.img') as src_img_path:
+
+    assert qemu_img('create', '-f', iotests.imgfmt, src_img_path, '64M') == 0
+    assert qemu_io_silent('-f', iotests.imgfmt, src_img_path,
+                          '-c', 'write -P 42 0M 64M') == 0
+
+    vm.launch()
+
+    ret = vm.qmp('object-add', qom_type='throttle-group', id='tg',
+                 props={'x-bps-read': 4096})
+    assert ret['return'] == {}
+
+    ret = vm.qmp('blockdev-add',
+                 node_name='source',
+                 driver=iotests.imgfmt,
+                 file={
+                     'driver': 'file',
+                     'filename': src_img_path
+                 })
+    assert ret['return'] == {}
+
+    ret = vm.qmp('blockdev-add',
+                 node_name='throttled-source',
+                 driver='throttle',
+                 throttle_group='tg',
+                 file='source')
+    assert ret['return'] == {}
+
+    ret = vm.qmp('blockdev-add',
+                 node_name='target',
+                 driver='null-co',
+                 size=(64 * 1048576))
+    assert ret['return'] == {}
+
+    ret = vm.qmp('blockdev-mirror',
+                 job_id='mirror',
+                 device='throttled-source',
+                 target='target',
+                 sync='full')
+    assert ret['return'] == {}
+
+    log(vm.qmp('quit'))
+
+    with iotests.Timeout(5, 'Timeout waiting for VM to quit'):
+        vm.shutdown(has_quit=True)
diff --git a/tests/qemu-iotests/218.out b/tests/qemu-iotests/218.out
index 825a657..5a86a97 100644
--- a/tests/qemu-iotests/218.out
+++ b/tests/qemu-iotests/218.out
@@ -28,3 +28,7 @@ Cancelling job
 Cancelling job
 {"return": {}}
 {"data": {"device": "mirror", "len": 1048576, "offset": 1048576, "speed": 0, "type": "mirror"}, "event": "BLOCK_JOB_CANCELLED", "timestamp": {"microseconds": "USECS", "seconds": "SECS"}}
+
+=== Cancel mirror job from throttled node by quitting ===
+
+{"return": {}}
diff --git a/tests/qemu-iotests/227 b/tests/qemu-iotests/227
index bdd727a..637d7c3 100755
--- a/tests/qemu-iotests/227
+++ b/tests/qemu-iotests/227
@@ -57,7 +57,7 @@ echo
 echo '=== blockstats with -drive if=virtio ==='
 echo
 
-run_qemu -drive driver=null-co,if=virtio <<EOF
+run_qemu -drive driver=null-co,read-zeroes=on,if=virtio <<EOF
 { "execute": "qmp_capabilities" }
 { "execute": "query-blockstats"}
 { "execute": "quit" }
@@ -87,7 +87,7 @@ echo
 echo '=== blockstats with -blockdev and -device ==='
 echo
 
-run_qemu -blockdev driver=null-co,node-name=null -device virtio-blk,drive=null,id=virtio0 <<EOF
+run_qemu -blockdev driver=null-co,read-zeroes=on,node-name=null -device virtio-blk,drive=null,id=virtio0 <<EOF
 { "execute": "qmp_capabilities" }
 { "execute": "query-blockstats"}
 { "execute": "quit" }
diff --git a/tests/qemu-iotests/227.out b/tests/qemu-iotests/227.out
index e77efaf..3dd3ca5 100644
--- a/tests/qemu-iotests/227.out
+++ b/tests/qemu-iotests/227.out
@@ -2,7 +2,7 @@ QA output created by 227
 
 === blockstats with -drive if=virtio ===
 
-Testing: -drive driver=null-co,if=virtio
+Testing: -drive driver=null-co,read-zeroes=on,if=virtio
 {
     QMP_VERSION
 }
@@ -150,7 +150,7 @@ Testing: -blockdev driver=null-co,node-name=null
 
 === blockstats with -blockdev and -device ===
 
-Testing: -blockdev driver=null-co,node-name=null -device virtio-blk,drive=null,id=virtio0
+Testing: -blockdev driver=null-co,read-zeroes=on,node-name=null -device virtio-blk,drive=null,id=virtio0
 {
     QMP_VERSION
 }
diff --git a/tests/qemu-iotests/238 b/tests/qemu-iotests/238
index 1c0a46f..08bc7e6 100755
--- a/tests/qemu-iotests/238
+++ b/tests/qemu-iotests/238
@@ -31,7 +31,7 @@ else:
 vm = iotests.VM()
 vm.launch()
 
-log(vm.qmp('blockdev-add', node_name='hd0', driver='null-co'))
+log(vm.qmp('blockdev-add', node_name='hd0', driver='null-co', read_zeroes=True))
 log(vm.qmp('object-add', qom_type='iothread', id='iothread0'))
 log(vm.qmp('device_add', id='scsi0', driver=virtio_scsi_device, iothread='iothread0'))
 log(vm.qmp('device_add', id='scsi-hd0', driver='scsi-hd', drive='hd0'))
diff --git a/tests/qemu-iotests/240 b/tests/qemu-iotests/240
index 5be6b9c..f73bc07 100755
--- a/tests/qemu-iotests/240
+++ b/tests/qemu-iotests/240
@@ -76,7 +76,7 @@ echo
 
 run_qemu <<EOF
 { "execute": "qmp_capabilities" }
-{ "execute": "blockdev-add", "arguments": {"driver": "null-co", "node-name": "hd0"}}
+{ "execute": "blockdev-add", "arguments": {"driver": "null-co", "read-zeroes": true, "node-name": "hd0"}}
 { "execute": "object-add", "arguments": {"qom-type": "iothread", "id": "iothread0"}}
 { "execute": "device_add", "arguments": {"id": "scsi0", "driver": "${virtio_scsi}", "iothread": "iothread0"}}
 { "execute": "device_add", "arguments": {"id": "scsi-hd0", "driver": "scsi-hd", "drive": "hd0"}}
@@ -94,7 +94,7 @@ echo
 
 run_qemu <<EOF
 { "execute": "qmp_capabilities" }
-{ "execute": "blockdev-add", "arguments": {"driver": "null-co", "node-name": "hd0", "read-only": true}}
+{ "execute": "blockdev-add", "arguments": {"driver": "null-co", "read-zeroes": true, "node-name": "hd0", "read-only": true}}
 { "execute": "object-add", "arguments": {"qom-type": "iothread", "id": "iothread0"}}
 { "execute": "device_add", "arguments": {"id": "scsi0", "driver": "${virtio_scsi}", "iothread": "iothread0"}}
 { "execute": "device_add", "arguments": {"id": "scsi-hd0", "driver": "scsi-hd", "drive": "hd0"}}
@@ -112,7 +112,7 @@ echo
 
 run_qemu <<EOF
 { "execute": "qmp_capabilities" }
-{ "execute": "blockdev-add", "arguments": {"driver": "null-co", "node-name": "hd0", "read-only": true}}
+{ "execute": "blockdev-add", "arguments": {"driver": "null-co", "read-zeroes": true, "node-name": "hd0", "read-only": true}}
 { "execute": "object-add", "arguments": {"qom-type": "iothread", "id": "iothread0"}}
 { "execute": "object-add", "arguments": {"qom-type": "iothread", "id": "iothread1"}}
 { "execute": "device_add", "arguments": {"id": "scsi0", "driver": "${virtio_scsi}", "iothread": "iothread0"}}
@@ -134,7 +134,7 @@ echo
 
 run_qemu <<EOF
 { "execute": "qmp_capabilities" }
-{ "execute": "blockdev-add", "arguments": {"driver": "null-co", "node-name": "hd0", "read-only": true}}
+{ "execute": "blockdev-add", "arguments": {"driver": "null-co", "read-zeroes": true, "node-name": "hd0", "read-only": true}}
 { "execute": "nbd-server-start", "arguments": {"addr":{"type":"unix","data":{"path":"$TEST_DIR/nbd"}}}}
 { "execute": "nbd-server-add", "arguments": {"device":"hd0"}}
 { "execute": "object-add", "arguments": {"qom-type": "iothread", "id": "iothread0"}}
diff --git a/tests/qemu-iotests/255 b/tests/qemu-iotests/255
index 49433ec..3632d50 100755
--- a/tests/qemu-iotests/255
+++ b/tests/qemu-iotests/255
@@ -132,4 +132,4 @@ with iotests.FilePath('src.qcow2') as src_path, \
     vm.qmp_log('block-job-cancel', device='job0')
     vm.qmp_log('quit')
 
-    vm.shutdown()
+    vm.shutdown(has_quit=True)
diff --git a/tests/test-bdrv-drain.c b/tests/test-bdrv-drain.c
index 12e2ecf..03fa114 100644
--- a/tests/test-bdrv-drain.c
+++ b/tests/test-bdrv-drain.c
@@ -1527,6 +1527,150 @@ static void test_set_aio_context(void)
     iothread_join(b);
 }
 
+
+typedef struct TestDropBackingBlockJob {
+    BlockJob common;
+    bool should_complete;
+    bool *did_complete;
+    BlockDriverState *detach_also;
+} TestDropBackingBlockJob;
+
+static int coroutine_fn test_drop_backing_job_run(Job *job, Error **errp)
+{
+    TestDropBackingBlockJob *s =
+        container_of(job, TestDropBackingBlockJob, common.job);
+
+    while (!s->should_complete) {
+        job_sleep_ns(job, 0);
+    }
+
+    return 0;
+}
+
+static void test_drop_backing_job_commit(Job *job)
+{
+    TestDropBackingBlockJob *s =
+        container_of(job, TestDropBackingBlockJob, common.job);
+
+    bdrv_set_backing_hd(blk_bs(s->common.blk), NULL, &error_abort);
+    bdrv_set_backing_hd(s->detach_also, NULL, &error_abort);
+
+    *s->did_complete = true;
+}
+
+static const BlockJobDriver test_drop_backing_job_driver = {
+    .job_driver = {
+        .instance_size  = sizeof(TestDropBackingBlockJob),
+        .free           = block_job_free,
+        .user_resume    = block_job_user_resume,
+        .drain          = block_job_drain,
+        .run            = test_drop_backing_job_run,
+        .commit         = test_drop_backing_job_commit,
+    }
+};
+
+/**
+ * Creates a child node with three parent nodes on it, and then runs a
+ * block job on the final one, parent-node-2.
+ *
+ * The job is then asked to complete before a section where the child
+ * is drained.
+ *
+ * Ending this section will undrain the child's parents, first
+ * parent-node-2, then parent-node-1, then parent-node-0 -- the parent
+ * list is in reverse order of how they were added.  Ending the drain
+ * on parent-node-2 will resume the job, thus completing it and
+ * scheduling job_exit().
+ *
+ * Ending the drain on parent-node-1 will poll the AioContext, which
+ * lets job_exit() and thus test_drop_backing_job_commit() run.  That
+ * function first removes the child as parent-node-2's backing file.
+ *
+ * In old (and buggy) implementations, there are two problems with
+ * that:
+ * (A) bdrv_drain_invoke() polls for every node that leaves the
+ *     drained section.  This means that job_exit() is scheduled
+ *     before the child has left the drained section.  Its
+ *     quiesce_counter is therefore still 1 when it is removed from
+ *     parent-node-2.
+ *
+ * (B) bdrv_replace_child_noperm() calls drained_end() on the old
+ *     child's parents as many times as the child is quiesced.  This
+ *     means it will call drained_end() on parent-node-2 once.
+ *     Because parent-node-2 is no longer quiesced at this point, this
+ *     will fail.
+ *
+ * bdrv_replace_child_noperm() therefore must call drained_end() on
+ * the parent only if it really is still drained because the child is
+ * drained.
+ *
+ * If removing child from parent-node-2 was successful (as it should
+ * be), test_drop_backing_job_commit() will then also remove the child
+ * from parent-node-0.
+ *
+ * With an old version of our drain infrastructure ((A) above), that
+ * resulted in the following flow:
+ *
+ * 1. child attempts to leave its drained section.  The call recurses
+ *    to its parents.
+ *
+ * 2. parent-node-2 leaves the drained section.  Polling in
+ *    bdrv_drain_invoke() will schedule job_exit().
+ *
+ * 3. parent-node-1 leaves the drained section.  Polling in
+ *    bdrv_drain_invoke() will run job_exit(), thus disconnecting
+ *    parent-node-0 from the child node.
+ *
+ * 4. bdrv_parent_drained_end() uses a QLIST_FOREACH_SAFE() loop to
+ *    iterate over the parents.  Thus, it now accesses the BdrvChild
+ *    object that used to connect parent-node-0 and the child node.
+ *    However, that object no longer exists, so it accesses a dangling
+ *    pointer.
+ *
+ * The solution is to only poll once when running a bdrv_drained_end()
+ * operation, specifically at the end when all drained_end()
+ * operations for all involved nodes have been scheduled.
+ * Note that this also solves (A) above, thus hiding (B).
+ */
+static void test_blockjob_commit_by_drained_end(void)
+{
+    BlockDriverState *bs_child, *bs_parents[3];
+    TestDropBackingBlockJob *job;
+    bool job_has_completed = false;
+    int i;
+
+    bs_child = bdrv_new_open_driver(&bdrv_test, "child-node", BDRV_O_RDWR,
+                                    &error_abort);
+
+    for (i = 0; i < 3; i++) {
+        char name[32];
+        snprintf(name, sizeof(name), "parent-node-%i", i);
+        bs_parents[i] = bdrv_new_open_driver(&bdrv_test, name, BDRV_O_RDWR,
+                                             &error_abort);
+        bdrv_set_backing_hd(bs_parents[i], bs_child, &error_abort);
+    }
+
+    job = block_job_create("job", &test_drop_backing_job_driver, NULL,
+                           bs_parents[2], 0, BLK_PERM_ALL, 0, 0, NULL, NULL,
+                           &error_abort);
+
+    job->detach_also = bs_parents[0];
+    job->did_complete = &job_has_completed;
+
+    job_start(&job->common.job);
+
+    job->should_complete = true;
+    bdrv_drained_begin(bs_child);
+    g_assert(!job_has_completed);
+    bdrv_drained_end(bs_child);
+    g_assert(job_has_completed);
+
+    bdrv_unref(bs_parents[0]);
+    bdrv_unref(bs_parents[1]);
+    bdrv_unref(bs_parents[2]);
+    bdrv_unref(bs_child);
+}
+
 int main(int argc, char **argv)
 {
     int ret;
@@ -1610,6 +1754,9 @@ int main(int argc, char **argv)
 
     g_test_add_func("/bdrv-drain/set_aio_context", test_set_aio_context);
 
+    g_test_add_func("/bdrv-drain/blockjob/commit_by_drained_end",
+                    test_blockjob_commit_by_drained_end);
+
     ret = g_test_run();
     qemu_event_destroy(&done_event);
     return ret;
diff --git a/tests/test-block-iothread.c b/tests/test-block-iothread.c
index 79d9cf8..1949d5e 100644
--- a/tests/test-block-iothread.c
+++ b/tests/test-block-iothread.c
@@ -348,8 +348,8 @@ static void test_sync_op(const void *opaque)
     if (t->blkfn) {
         t->blkfn(blk);
     }
-    aio_context_release(ctx);
     blk_set_aio_context(blk, qemu_get_aio_context(), &error_abort);
+    aio_context_release(ctx);
 
     bdrv_unref(bs);
     blk_unref(blk);
@@ -476,6 +476,7 @@ static void test_propagate_basic(void)
 {
     IOThread *iothread = iothread_new();
     AioContext *ctx = iothread_get_aio_context(iothread);
+    AioContext *main_ctx;
     BlockBackend *blk;
     BlockDriverState *bs_a, *bs_b, *bs_verify;
     QDict *options;
@@ -504,12 +505,14 @@ static void test_propagate_basic(void)
     g_assert(bdrv_get_aio_context(bs_b) == ctx);
 
     /* Switch the AioContext back */
-    ctx = qemu_get_aio_context();
-    blk_set_aio_context(blk, ctx, &error_abort);
-    g_assert(blk_get_aio_context(blk) == ctx);
-    g_assert(bdrv_get_aio_context(bs_a) == ctx);
-    g_assert(bdrv_get_aio_context(bs_verify) == ctx);
-    g_assert(bdrv_get_aio_context(bs_b) == ctx);
+    main_ctx = qemu_get_aio_context();
+    aio_context_acquire(ctx);
+    blk_set_aio_context(blk, main_ctx, &error_abort);
+    aio_context_release(ctx);
+    g_assert(blk_get_aio_context(blk) == main_ctx);
+    g_assert(bdrv_get_aio_context(bs_a) == main_ctx);
+    g_assert(bdrv_get_aio_context(bs_verify) == main_ctx);
+    g_assert(bdrv_get_aio_context(bs_b) == main_ctx);
 
     bdrv_unref(bs_verify);
     bdrv_unref(bs_b);
@@ -534,6 +537,7 @@ static void test_propagate_diamond(void)
 {
     IOThread *iothread = iothread_new();
     AioContext *ctx = iothread_get_aio_context(iothread);
+    AioContext *main_ctx;
     BlockBackend *blk;
     BlockDriverState *bs_a, *bs_b, *bs_c, *bs_verify;
     QDict *options;
@@ -573,13 +577,15 @@ static void test_propagate_diamond(void)
     g_assert(bdrv_get_aio_context(bs_c) == ctx);
 
     /* Switch the AioContext back */
-    ctx = qemu_get_aio_context();
-    blk_set_aio_context(blk, ctx, &error_abort);
-    g_assert(blk_get_aio_context(blk) == ctx);
-    g_assert(bdrv_get_aio_context(bs_verify) == ctx);
-    g_assert(bdrv_get_aio_context(bs_a) == ctx);
-    g_assert(bdrv_get_aio_context(bs_b) == ctx);
-    g_assert(bdrv_get_aio_context(bs_c) == ctx);
+    main_ctx = qemu_get_aio_context();
+    aio_context_acquire(ctx);
+    blk_set_aio_context(blk, main_ctx, &error_abort);
+    aio_context_release(ctx);
+    g_assert(blk_get_aio_context(blk) == main_ctx);
+    g_assert(bdrv_get_aio_context(bs_verify) == main_ctx);
+    g_assert(bdrv_get_aio_context(bs_a) == main_ctx);
+    g_assert(bdrv_get_aio_context(bs_b) == main_ctx);
+    g_assert(bdrv_get_aio_context(bs_c) == main_ctx);
 
     blk_unref(blk);
     bdrv_unref(bs_verify);
@@ -685,7 +691,9 @@ static void test_attach_second_node(void)
     g_assert(bdrv_get_aio_context(bs) == ctx);
     g_assert(bdrv_get_aio_context(filter) == ctx);
 
+    aio_context_acquire(ctx);
     blk_set_aio_context(blk, main_ctx, &error_abort);
+    aio_context_release(ctx);
     g_assert(blk_get_aio_context(blk) == main_ctx);
     g_assert(bdrv_get_aio_context(bs) == main_ctx);
     g_assert(bdrv_get_aio_context(filter) == main_ctx);
@@ -712,7 +720,9 @@ static void test_attach_preserve_blk_ctx(void)
     g_assert(bdrv_get_aio_context(bs) == ctx);
 
     /* Remove the node again */
+    aio_context_acquire(ctx);
     blk_remove_bs(blk);
+    aio_context_release(ctx);
     g_assert(blk_get_aio_context(blk) == ctx);
     g_assert(bdrv_get_aio_context(bs) == qemu_get_aio_context());
 
@@ -721,7 +731,9 @@ static void test_attach_preserve_blk_ctx(void)
     g_assert(blk_get_aio_context(blk) == ctx);
     g_assert(bdrv_get_aio_context(bs) == ctx);
 
+    aio_context_acquire(ctx);
     blk_set_aio_context(blk, qemu_get_aio_context(), &error_abort);
+    aio_context_release(ctx);
     bdrv_unref(bs);
     blk_unref(blk);
 }
diff --git a/vl.c b/vl.c
index a5808f9..53335a5 100644
--- a/vl.c
+++ b/vl.c
@@ -4480,6 +4480,17 @@ int main(int argc, char **argv, char **envp)
      */
     migration_shutdown();
 
+    /*
+     * We must cancel all block jobs while the block layer is drained,
+     * or cancelling will be affected by throttling and thus may block
+     * for an extended period of time.
+     * vm_shutdown() will bdrv_drain_all(), so we may as well include
+     * it in the drained section.
+     * We do not need to end this section, because we do not want any
+     * requests happening from here on anyway.
+     */
+    bdrv_drain_all_begin();
+
     /* No more vcpu or device emulation activity beyond this point */
     vm_shutdown();