block: stop relying on io_flush() in bdrv_drain_all()

If a block driver has no file descriptors to monitor but there are still
active requests, it can return 1 from .io_flush().  This is used to spin
during synchronous I/O.

Stop relying on .io_flush() and instead check
QLIST_EMPTY(&bs->tracked_requests) to decide whether there are active
requests.

This is the first step in removing .io_flush() so that event loops no
longer need to have the concept of synchronous I/O.  Eventually we may
be able to kill synchronous I/O completely by running everything in a
coroutine, but that is future work.

Note this patch moves bs->throttled_reqs initialization to bdrv_new() so
that bdrv_requests_pending(bs) can safely access it.  In practice bs is
g_malloc0() so the memory is already zeroed but it's safer to initialize
the queue properly.

We also need to fix up block/stream.c:close_unused_images() to prevent
traversing a dangling pointer while it rearranges the backing file
chain.  This is necessary since the new bdrv_drain_all() traverses the
backing file chain.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/block/stream.c b/block/stream.c
index 7fe9e48..db49b4d 100644
--- a/block/stream.c
+++ b/block/stream.c
@@ -57,6 +57,11 @@ static void close_unused_images(BlockDriverState *top, BlockDriverState *base,
     BlockDriverState *intermediate;
     intermediate = top->backing_hd;
 
+    /* Must assign before bdrv_delete() to prevent traversing dangling pointer
+     * while we delete backing image instances.
+     */
+    top->backing_hd = base;
+
     while (intermediate) {
         BlockDriverState *unused;
 
@@ -70,7 +75,6 @@ static void close_unused_images(BlockDriverState *top, BlockDriverState *base,
         unused->backing_hd = NULL;
         bdrv_delete(unused);
     }
-    top->backing_hd = base;
 }
 
 static void coroutine_fn stream_run(void *opaque)