From 49d9c9dced7278517105e9cfec34ea4af716432d Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Thu, 9 Jun 2022 17:47:12 +0100 Subject: [PATCH 6/6] linux-aio: explain why max batch is checked in laio_io_unplug() RH-Author: Stefan Hajnoczi RH-MergeRequest: 102: linux-aio: fix unbalanced plugged counter in laio_io_unplug() RH-Commit: [2/2] b3d6421086bde50d4baad2343b2df89c5f66950e (stefanha/centos-stream-qemu-kvm) RH-Bugzilla: 2092788 RH-Acked-by: Miroslav Rezanina RH-Acked-by: Kevin Wolf RH-Acked-by: Stefano Garzarella It may not be obvious why laio_io_unplug() checks max batch. I discussed this with Stefano and have added a comment summarizing the reason. Cc: Stefano Garzarella Cc: Kevin Wolf Signed-off-by: Stefan Hajnoczi Reviewed-by: Stefano Garzarella Message-id: 20220609164712.1539045-3-stefanha@redhat.com Signed-off-by: Stefan Hajnoczi (cherry picked from commit 99b969fbe105117f5af6060d3afef40ca39cc9c1) Signed-off-by: Stefan Hajnoczi --- block/linux-aio.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/block/linux-aio.c b/block/linux-aio.c index 6078da7e42..9c2393a2f7 100644 --- a/block/linux-aio.c +++ b/block/linux-aio.c @@ -365,6 +365,12 @@ void laio_io_unplug(BlockDriverState *bs, LinuxAioState *s, assert(s->io_q.plugged); s->io_q.plugged--; + /* + * Why max batch checking is performed here: + * Another BDS may have queued requests with a higher dev_max_batch and + * therefore in_queue could now exceed our dev_max_batch. Re-check the max + * batch so we can honor our device's dev_max_batch. + */ if (s->io_q.in_queue >= laio_max_batch(s, dev_max_batch) || (!s->io_q.plugged && !s->io_q.blocked && !QSIMPLEQ_EMPTY(&s->io_q.pending))) { -- 2.31.1