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