On 12/06/2017 08:45 AM, Stefan Hajnoczi wrote:
> bdrv_unref() requires the AioContext lock because bdrv_flush() uses
> BDRV_POLL_WHILE(), which assumes the AioContext is currently held.  If
> BDRV_POLL_WHILE() runs without AioContext held the
> pthread_mutex_unlock() call in aio_context_release() fails.
> 
> This patch moves bdrv_unref() into the AioContext locked region to solve
> the following pthread_mutex_unlock() failure:
> 

> diff --git a/blockdev.c b/blockdev.c
> index 56a6b24a0b..3c8d994ced 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -1812,8 +1812,8 @@ static void external_snapshot_clean(BlkActionState *common)
>                               DO_UPCAST(ExternalSnapshotState, common, common);
>      if (state->aio_context) {
>          bdrv_drained_end(state->old_bs);
> -        aio_context_release(state->aio_context);
>          bdrv_unref(state->new_bs);
> +        aio_context_release(state->aio_context);

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org