From 187eb7a418af93375e42298d06e231e2bec3cf00 Mon Sep 17 00:00:00 2001
From: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Date: Thu, 9 Mar 2023 08:15:42 -0500
Subject: [PATCH 10/13] async: clarify usage of barriers in the polling case

RH-Author: Emanuele Giuseppe Esposito <eesposit@redhat.com>
RH-MergeRequest: 263: qatomic: add smp_mb__before/after_rmw()
RH-Bugzilla: 2168472
RH-Acked-by: Cornelia Huck <cohuck@redhat.com>
RH-Acked-by: Eric Auger <eric.auger@redhat.com>
RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
RH-Acked-by: David Hildenbrand <david@redhat.com>
RH-Commit: [10/10] 3be07ccc6137a0336becfe63a818d9cbadb38e9c

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2168472

commit 6229438cca037d42f44a96d38feb15cb102a444f
Author: Paolo Bonzini <pbonzini@redhat.com>
Date:   Mon Mar 6 10:43:52 2023 +0100

    async: clarify usage of barriers in the polling case

    Explain that aio_context_notifier_poll() relies on
    aio_notify_accept() to catch all the memory writes that were
    done before ctx->notified was set to true.

    Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
    Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 util/async.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/util/async.c b/util/async.c
index 795fe699b6..2a63bf90f2 100644
--- a/util/async.c
+++ b/util/async.c
@@ -463,8 +463,9 @@ void aio_notify_accept(AioContext *ctx)
     qatomic_set(&ctx->notified, false);
 
     /*
-     * Write ctx->notified before reading e.g. bh->flags.  Pairs with smp_wmb
-     * in aio_notify.
+     * Order reads of ctx->notified (in aio_context_notifier_poll()) and the
+     * above clearing of ctx->notified before reads of e.g. bh->flags.  Pairs
+     * with smp_wmb() in aio_notify.
      */
     smp_mb();
 }
@@ -487,6 +488,11 @@ static bool aio_context_notifier_poll(void *opaque)
     EventNotifier *e = opaque;
     AioContext *ctx = container_of(e, AioContext, notifier);
 
+    /*
+     * No need for load-acquire because we just want to kick the
+     * event loop.  aio_notify_accept() takes care of synchronizing
+     * the event loop with the producers.
+     */
     return qatomic_read(&ctx->notified);
 }
 
-- 
2.37.3