[PULL 2/3] async: always set ctx->notified in aio_notify()

[Stefan Hajnoczi <stefanha@redhat.com>]

aio_notify() does not set ctx->notified when called with
ctx->aio_notify_me disabled. Therefore aio_notify_me needs to be enabled
during polling.

This is suboptimal since expensive event_notifier_set(&ctx->notifier)
and event_notifier_test_and_clear(&ctx->notifier) calls are required
when ctx->aio_notify_me is enabled.

Change aio_notify() so that aio->notified is always set, regardless of
ctx->aio_notify_me. This will make polling cheaper since
ctx->aio_notify_me can remain disabled. Move the
event_notifier_test_and_clear() to the fd handler function (which is now
no longer an empty function so "dummy" has been dropped from its name).

The next patch takes advantage of this by optimizing polling in
util/aio-posix.c.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200806131802.569478-3-stefanha@redhat.com

[Paolo Bonzini pointed out that the smp_wmb() in aio_notify_accept()
should be smp_wb() but the comment should be smp_wmb() instead of
smp_wb(). Fixed.
--Stefan]

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 util/async.c | 32 +++++++++++++++++++++-----------
 1 file changed, 21 insertions(+), 11 deletions(-)

diff --git a/util/async.c b/util/async.c
index d9f987e133..4266745dee 100644
--- a/util/async.c
+++ b/util/async.c
@@ -419,25 +419,32 @@ LuringState *aio_get_linux_io_uring(AioContext *ctx)
 
 void aio_notify(AioContext *ctx)
 {
-    /* Write e.g. bh->scheduled before reading ctx->notify_me.  Pairs
+    /*
+     * Write e.g. bh->flags before writing ctx->notified.  Pairs with smp_mb in
+     * aio_notify_accept.
+     */
+    smp_wmb();
+    atomic_set(&ctx->notified, true);
+
+    /*
+     * Write ctx->notified before reading ctx->notify_me.  Pairs
      * with smp_mb in aio_ctx_prepare or aio_poll.
      */
     smp_mb();
     if (atomic_read(&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)
-#ifdef WIN32
-        || true
-#endif
-    ) {
-        event_notifier_test_and_clear(&ctx->notifier);
-    }
+    atomic_set(&ctx->notified, false);
+
+    /*
+     * Write ctx->notified before reading e.g. bh->flags.  Pairs with smp_wmb
+     * in aio_notify.
+     */
+    smp_mb();
 }
 
 static void aio_timerlist_notify(void *opaque, QEMUClockType type)
@@ -445,8 +452,11 @@ static void aio_timerlist_notify(void *opaque, QEMUClockType type)
     aio_notify(opaque);
 }
 
-static void aio_context_notifier_dummy_cb(EventNotifier *e)
+static void aio_context_notifier_cb(EventNotifier *e)
 {
+    AioContext *ctx = container_of(e, AioContext, notifier);
+
+    event_notifier_test_and_clear(&ctx->notifier);
 }
 
 /* Returns true if aio_notify() was called (e.g. a BH was scheduled) */
@@ -508,7 +518,7 @@ AioContext *aio_context_new(Error **errp)
 
     aio_set_event_notifier(ctx, &ctx->notifier,
                            false,
-                           aio_context_notifier_dummy_cb,
+                           aio_context_notifier_cb,
                            aio_context_notifier_poll);
 #ifdef CONFIG_LINUX_AIO
     ctx->linux_aio = NULL;
-- 
2.26.2