block: Poll after drain on attaching a node

Commit dcf94a23b1 ('block: Don't poll in parent drain callbacks')
removed polling in bdrv_child_cb_drained_begin() on the grounds that the
original bdrv_drain() already will poll and BdrvChildRole.drained_begin
calls must not cause graph changes (and therefore must not call
aio_poll() or the recursion through the graph will break.

This reasoning is correct for calls through bdrv_do_drained_begin().
However, BdrvChildRole.drained_begin is also called when a node that is
already in a drained section (i.e. bdrv_do_drained_begin() has already
returned and therefore can't poll any more) is attached to a new parent.
In this case, we must explicitly poll to have all requests completed
before the drained new child can be attached to the parent.

In bdrv_replace_child_noperm(), we know that we're not inside the
recursion of bdrv_do_drained_begin() because graph changes are not
allowed there, and bdrv_replace_child_noperm() is a graph change. The
call of BdrvChildRole.drained_begin() must therefore be followed by a
BDRV_POLL_WHILE() that waits for the completion of requests.

Reported-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 20 insertions, 6 deletions

diff --git a/block/io.c b/block/io.c
index 1a2272f..038449f 100644
--- a/block/io.c
+++ b/block/io.c
@@ -52,9 +52,7 @@ void bdrv_parent_drained_begin(BlockDriverState *bs, BdrvChild *ignore,
         if (c == ignore || (ignore_bds_parents && c->role->parent_is_bds)) {
             continue;
         }
-        if (c->role->drained_begin) {
-            c->role->drained_begin(c);
-        }
+        bdrv_parent_drained_begin_single(c, false);
     }
 }
 
@@ -73,6 +71,14 @@ void bdrv_parent_drained_end(BlockDriverState *bs, BdrvChild *ignore,
     }
 }
 
+static bool bdrv_parent_drained_poll_single(BdrvChild *c)
+{
+    if (c->role->drained_poll) {
+        return c->role->drained_poll(c);
+    }
+    return false;
+}
+
 static bool bdrv_parent_drained_poll(BlockDriverState *bs, BdrvChild *ignore,
                                      bool ignore_bds_parents)
 {
@@ -83,14 +89,22 @@ static bool bdrv_parent_drained_poll(BlockDriverState *bs, BdrvChild *ignore,
         if (c == ignore || (ignore_bds_parents && c->role->parent_is_bds)) {
             continue;
         }
-        if (c->role->drained_poll) {
-            busy |= c->role->drained_poll(c);
-        }
+        busy |= bdrv_parent_drained_poll_single(c);
     }
 
     return busy;
 }
 
+void bdrv_parent_drained_begin_single(BdrvChild *c, bool poll)
+{
+    if (c->role->drained_begin) {
+        c->role->drained_begin(c);
+    }
+    if (poll) {
+        BDRV_POLL_WHILE(c->bs, bdrv_parent_drained_poll_single(c));
+    }
+}
+
 static void bdrv_merge_limits(BlockLimits *dst, const BlockLimits *src)
 {
     dst->opt_transfer = MAX(dst->opt_transfer, src->opt_transfer);