From 8eeaa706ba73251063cb80d87ae838d2d5b08e9a Mon Sep 17 00:00:00 2001 From: Kevin Wolf Date: Wed, 19 Nov 2025 18:27:20 +0100 Subject: [PATCH] block-backend: Fix race when resuming queued requests 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_inc_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 Signed-off-by: Kevin Wolf Message-ID: <20251119172720.135424-1-kwolf@redhat.com> Reviewed-by: Hanna Czenczek Tested-by: Andrey Drobyshev Reviewed-by: Fiona Ebner Signed-off-by: Kevin Wolf --- block/block-backend.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) diff --git a/block/block-backend.c b/block/block-backend.c index f8d6ba65c11..d6df3691887 100644 --- a/block/block-backend.c +++ b/block/block-backend.c @@ -1318,9 +1318,9 @@ static void coroutine_fn blk_wait_while_drained(BlockBackend *blk) * section. */ qemu_mutex_lock(&blk->queued_requests_lock); + /* blk_root_drained_end() has the corresponding blk_inc_in_flight() */ blk_dec_in_flight(blk); qemu_co_queue_wait(&blk->queued_requests, &blk->queued_requests_lock); - blk_inc_in_flight(blk); qemu_mutex_unlock(&blk->queued_requests_lock); } } @@ -2767,9 +2767,11 @@ static void blk_root_drained_end(BdrvChild *child) blk->dev_ops->drained_end(blk->dev_opaque); } qemu_mutex_lock(&blk->queued_requests_lock); - while (qemu_co_enter_next(&blk->queued_requests, - &blk->queued_requests_lock)) { + while (!qemu_co_queue_empty(&blk->queued_requests)) { /* Resume all queued requests */ + blk_inc_in_flight(blk); + qemu_co_enter_next(&blk->queued_requests, + &blk->queued_requests_lock); } qemu_mutex_unlock(&blk->queued_requests_lock); } -- 2.47.3