[PULL 17/18] linux-aio: fix unbalanced plugged counter in laio_io_unplug()

From: Stefan Hajnoczi <stefanha@redhat.com>
To: qemu-devel@nongnu.org
Cc: "Thomas Huth" <thuth@redhat.com>,
	"Jagannathan Raman" <jag.raman@oracle.com>,
	"Aarushi Mehta" <mehta.aaru20@gmail.com>,
	"Philippe Mathieu-Daudé" <f4bug@amsat.org>,
	"Elena Ufimtseva" <elena.ufimtseva@oracle.com>,
	"Marcel Apfelbaum" <marcel.apfelbaum@gmail.com>,
	"Stefan Hajnoczi" <stefanha@redhat.com>,
	"Laurent Vivier" <lvivier@redhat.com>,
	qemu-block@nongnu.org,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	"Richard Henderson" <richard.henderson@linaro.org>,
	virtio-fs@redhat.com, "Hanna Reitz" <hreitz@redhat.com>,
	"David Hildenbrand" <david@redhat.com>,
	"Alex Bennée" <alex.bennee@linaro.org>,
	"Eric Blake" <eblake@redhat.com>, "Kevin Wolf" <kwolf@redhat.com>,
	"Daniel P. Berrangé" <berrange@redhat.com>,
	"Beraldo Leal" <bleal@redhat.com>, "Peter Xu" <peterx@redhat.com>,
	"Eduardo Habkost" <eduardo@habkost.net>,
	"Qiuhao Li" <Qiuhao.Li@outlook.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Markus Armbruster" <armbru@redhat.com>,
	"Bandan Das" <bsd@redhat.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	"Stefano Garzarella" <sgarzare@redhat.com>,
	"Alexander Bulekov" <alxndr@bu.edu>,
	"Julia Suvorova" <jusual@redhat.com>,
	"Darren Kenny" <darren.kenny@oracle.com>,
	"Wainer dos Santos Moschetta" <wainersm@redhat.com>,
	"John G Johnson" <john.g.johnson@oracle.com>,
	"Nikolay Tenev" <nt@storpool.com>
Subject: [PULL 17/18] linux-aio: fix unbalanced plugged counter in laio_io_unplug()
Date: Wed, 15 Jun 2022 16:51:28 +0100	[thread overview]
Message-ID: <20220615155129.1025811-18-stefanha@redhat.com> (raw)
In-Reply-To: <20220615155129.1025811-1-stefanha@redhat.com>

Every laio_io_plug() call has a matching laio_io_unplug() call. There is
a plugged counter that tracks the number of levels of plugging and
allows for nesting.

The plugged counter must reflect the balance between laio_io_plug() and
laio_io_unplug() calls accurately. Otherwise I/O stalls occur since
io_submit(2) calls are skipped while plugged.

Reported-by: Nikolay Tenev <nt@storpool.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-id: 20220609164712.1539045-2-stefanha@redhat.com
Cc: Stefano Garzarella <sgarzare@redhat.com>
Fixes: 68d7946648 ("linux-aio: add `dev_max_batch` parameter to laio_io_unplug()")
[Stefano Garzarella suggested adding a Fixes tag.
--Stefan]
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 block/linux-aio.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/block/linux-aio.c b/block/linux-aio.c
index 4c423fcccf..6078da7e42 100644
--- a/block/linux-aio.c
+++ b/block/linux-aio.c
@@ -363,8 +363,10 @@ void laio_io_unplug(BlockDriverState *bs, LinuxAioState *s,
                     uint64_t dev_max_batch)
 {
     assert(s->io_q.plugged);
+    s->io_q.plugged--;
+
     if (s->io_q.in_queue >= laio_max_batch(s, dev_max_batch) ||
-        (--s->io_q.plugged == 0 &&
+        (!s->io_q.plugged &&
          !s->io_q.blocked && !QSIMPLEQ_EMPTY(&s->io_q.pending))) {
         ioq_submit(s);
     }
-- 
2.36.1


