[Qemu-block] [RFC PATCH] async: Fix aio_notify_accept

[Fam Zheng]

>From main loop, bdrv_set_aio_context() can call IOThread's aio_poll().
That breaks aio_notify() because the ctx->notifier event can get cleared
too early by this which causes IOThread hanging.

See https://bugzilla.redhat.com/show_bug.cgi?id=1562750 for details.

Signed-off-by: Fam Zheng <address@hidden>
---
 util/async.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/util/async.c b/util/async.c
index 05979f8014..c6e8aebc3a 100644
--- a/util/async.c
+++ b/util/async.c
@@ -355,7 +355,11 @@ void aio_notify(AioContext *ctx)
 
 void aio_notify_accept(AioContext *ctx)
 {
-    if (atomic_xchg(&ctx->notified, false)) {
+    /* If ctx->notify_me >= 2, another aio_poll() is waiting which may need the
+     * ctx->notifier event to wake up, so don't already clear it just because 
"we" are
+     * done iterating. */
+    if (atomic_read(&ctx->notify_me) < 2
+        && atomic_xchg(&ctx->notified, false)) {
         event_notifier_test_and_clear(&ctx->notifier);
     }
 }
-- 
2.17.1