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

[Stefan Hajnoczi <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