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From 3d823dda6832b76fd3d776131008107b0b0f7166 Mon Sep 17 00:00:00 2001
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From: Emanuele Giuseppe Esposito <eesposit@redhat.com>
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Date: Thu, 9 Mar 2023 08:24:36 -0500
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Subject: [PATCH 12/12] async: clarify usage of barriers in the polling case
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RH-Author: Emanuele Giuseppe Esposito <eesposit@redhat.com>
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RH-MergeRequest: 158: qatomic: add smp_mb__before/after_rmw()
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RH-Bugzilla: 2175660
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RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
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RH-Acked-by: Cornelia Huck <cohuck@redhat.com>
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RH-Acked-by: David Hildenbrand <david@redhat.com>
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RH-Acked-by: Eric Auger <eric.auger@redhat.com>
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RH-Commit: [9/9] b4ea298d75a75bb61e07a27d1296e0095fbc2bbf (eesposit/qemu-kvm)
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Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2175660
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commit 6229438cca037d42f44a96d38feb15cb102a444f
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Author: Paolo Bonzini <pbonzini@redhat.com>
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Date: Mon Mar 6 10:43:52 2023 +0100
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async: clarify usage of barriers in the polling case
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Explain that aio_context_notifier_poll() relies on
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aio_notify_accept() to catch all the memory writes that were
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done before ctx->notified was set to true.
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Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
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Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
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Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
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---
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util/async.c | 10 ++++++++--
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1 file changed, 8 insertions(+), 2 deletions(-)
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diff --git a/util/async.c b/util/async.c
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index 37d3e6036d..e0846baf93 100644
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--- a/util/async.c
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+++ b/util/async.c
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@@ -472,8 +472,9 @@ void aio_notify_accept(AioContext *ctx)
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qatomic_set(&ctx->notified, false);
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/*
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- * Write ctx->notified before reading e.g. bh->flags. Pairs with smp_wmb
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- * in aio_notify.
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+ * Order reads of ctx->notified (in aio_context_notifier_poll()) and the
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+ * above clearing of ctx->notified before reads of e.g. bh->flags. Pairs
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+ * with smp_wmb() in aio_notify.
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*/
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smp_mb();
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}
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@@ -496,6 +497,11 @@ static bool aio_context_notifier_poll(void *opaque)
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EventNotifier *e = opaque;
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AioContext *ctx = container_of(e, AioContext, notifier);
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+ /*
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+ * No need for load-acquire because we just want to kick the
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+ * event loop. aio_notify_accept() takes care of synchronizing
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+ * the event loop with the producers.
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+ */
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return qatomic_read(&ctx->notified);
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}
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--
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2.39.1
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