Re: bdrv_drained_begin deadlock with io-threads

[Dietmar Maurer <dietmar@proxmox.com>]

> On April 3, 2020 10:47 AM Kevin Wolf <kwolf@redhat.com> wrote:
> 
>  
> Am 03.04.2020 um 10:26 hat Dietmar Maurer geschrieben:
> > > With the following patch, it seems to survive for now. I'll give it some
> > > more testing tomorrow (also qemu-iotests to check that I didn't
> > > accidentally break something else.)
> > 
> > Wow, that was fast! Seems your patch fixes the bug!
> > 
> > I wonder what commit introduced that problem, maybe:
> > 
> > https://github.com/qemu/qemu/commit/cf3129323f900ef5ddbccbe86e4fa801e88c566e#diff-7cb66df56045598b75a219eebc27efb6
> > 
> > If so, version 4.1.X in not affected by this bug, but 4.2.0 and later?
> 
> Yes, I'm pretty sure that's the one.

I also wonder if we can loose an aio_wait_kick() by directly accessing blk->in_flight.
I thought this should use atomic_mb_read()?

diff --git a/block/block-backend.c b/block/block-backend.c
index 8b8f2a80a0..48f3721505 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -2275,7 +2275,7 @@ static bool blk_root_drained_poll(BdrvChild *child)
 {
     BlockBackend *blk = child->opaque;
     assert(blk->quiesce_counter);
-    return !!blk->in_flight;
+    return !!atomic_mb_read(&blk->in_flight);
 }
 
 static void blk_root_drained_end(BdrvChild *child, int *drained_end_counter)