[Qemu-devel] [PATCH v3] util/async: use atomic_mb_set in qemu_bh_cancel

[Sergio Lopez <slp@redhat.com>]

Commit b7a745d added a qemu_bh_cancel call to the completion function
as an optimization to prevent it from unnecessarily rescheduling itself.

This completion function is scheduled from worker_thread, after setting
the state of a ThreadPoolElement to THREAD_DONE.

This was considered to be safe, as the completion function restarts the
loop just after the call to qemu_bh_cancel. But, under certain access
patterns and scheduling conditions, the loop may wrongly use a
pre-fetched elem->state value, reading it as THREAD_QUEUED, and ending
the completion function without having processed a pending TPE linked at
pool->head:

         worker thread             |            I/O thread
------------------------------------------------------------------------
                                   | speculatively read req->state
req->state = THREAD_DONE;          |
qemu_bh_schedule(p->completion_bh) |
  bh->scheduled = 1;               |
                                   | qemu_bh_cancel(p->completion_bh)
                                   |   bh->scheduled = 0;
                                   | if (req->state == THREAD_DONE)
                                   |   // sees THREAD_QUEUED

The source of the misunderstanding was that qemu_bh_cancel is now being
used by the _consumer_ rather than the producer, and therefore now needs
to have acquire semantics just like e.g. aio_bh_poll.

In some situations, if there are no other independent requests in the
same aio context that could eventually trigger the scheduling of the
completion function, the omitted TPE and all operations pending on it
will get stuck forever.

Signed-off-by: Sergio Lopez <slp@redhat.com>
---
 util/async.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/util/async.c b/util/async.c
index 355af73ee7..0e1bd8780a 100644
--- a/util/async.c
+++ b/util/async.c
@@ -174,7 +174,7 @@ void qemu_bh_schedule(QEMUBH *bh)
  */
 void qemu_bh_cancel(QEMUBH *bh)
 {
-    bh->scheduled = 0;
+    atomic_mb_set(&bh->scheduled, 0);
 }
 
 /* This func is async.The bottom half will do the delete action at the finial
-- 
2.13.6