Re: [PATCH] block-backend: Fix race when resuming queued requests

[Hanna Czenczek <hreitz@redhat.com>]

On 19.11.25 18:27, Kevin Wolf wrote:
> When new requests arrive at a BlockBackend that is currently drained,
> these requests are queued until the drain section ends.
>
> There is a race window between blk_root_drained_end() waking up a queued
> request in an iothread from the main thread and blk_wait_while_drained()
> actually being woken up in the iothread and calling blk_in_flight(). If
> the BlockBackend is drained again during this window, drain won't wait
> for this request and it will sneak in when the BlockBackend is already
> supposed to be quiesced. This causes assertion failures in
> bdrv_drain_all_begin() and can have other unintended consequences.
>
> Fix this by increasing the in_flight counter immediately when scheduling
> the request to be resumed so that the next drain will wait for it to
> complete.
>
> Cc: qemu-stable@nongnu.org
> Reported-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>   block/block-backend.c | 8 +++++---
>   1 file changed, 5 insertions(+), 3 deletions(-)

Reviewed-by: Hanna Czenczek <hreitz@redhat.com>