[Qemu-stable] [PATCH] blockdev: use drained_begin/end for qmp_block_resize

[John Snow]

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1447551

If one tries to issue a block_resize while a guest is busy
accessing the disk, it is possible that qemu may deadlock
when invoking aio_poll from both the main loop and the iothread.

Replace another instance of bdrv_drain_all that doesn't
quite belong.

Suggested-by: Paolo Bonzini <address@hidden>
Signed-off-by: John Snow <address@hidden>
---
 blockdev.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/blockdev.c b/blockdev.c
index 0b38c3d..9ff06f3 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -2923,10 +2923,9 @@ void qmp_block_resize(bool has_device, const char 
*device,
         goto out;
     }
 
-    /* complete all in-flight operations before resizing the device */
-    bdrv_drain_all();
-
+    bdrv_drained_begin(bs);
     ret = blk_truncate(blk, size, errp);
+    bdrv_drained_end(bs);
 
 out:
     blk_unref(blk);
-- 
2.9.3