AioContext: optimize clearing the EventNotifier

It is pretty rare for aio_notify to actually set the EventNotifier.  It
can happen with worker threads such as thread-pool.c's, but otherwise it
should never be set thanks to the ctx->notify_me optimization.  The
previous patch, unfortunately, added an unconditional call to
event_notifier_test_and_clear; now add a userspace fast path that
avoids the call.

Note that it is not possible to do the same with event_notifier_set;
it would break, as proved (again) by the included formal model.

This patch survived over 3000 reboots on aarch64 KVM.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Message-id: 1437487673-23740-7-git-send-email-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/async.c b/async.c
index d625e8a803..9a98a74acb 100644
--- a/async.c
+++ b/async.c
@@ -203,7 +203,7 @@ aio_ctx_check(GSource *source)
     QEMUBH *bh;
 
     atomic_and(&ctx->notify_me, ~1);
-    event_notifier_test_and_clear(&ctx->notifier);
+    aio_notify_accept(ctx);
 
     for (bh = ctx->first_bh; bh; bh = bh->next) {
         if (!bh->deleted && bh->scheduled) {
@@ -267,6 +267,14 @@ void aio_notify(AioContext *ctx)
     smp_mb();
     if (ctx->notify_me) {
         event_notifier_set(&ctx->notifier);
+        atomic_mb_set(&ctx->notified, true);
+    }
+}
+
+void aio_notify_accept(AioContext *ctx)
+{
+    if (atomic_xchg(&ctx->notified, false)) {
+        event_notifier_test_and_clear(&ctx->notifier);
     }
 }