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From d7eae0ff4c7f7f7bf10f10272adf7c6971c0db9b 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 09:26:35 -0500
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Subject: [PATCH 01/13] aio_wait_kick: add missing memory barrier
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RH-Author: Emanuele Giuseppe Esposito <eesposit@redhat.com>
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RH-MergeRequest: 263: qatomic: add smp_mb__before/after_rmw()
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RH-Bugzilla: 2168472
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RH-Acked-by: Cornelia Huck <cohuck@redhat.com>
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RH-Acked-by: Eric Auger <eric.auger@redhat.com>
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RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
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RH-Acked-by: David Hildenbrand <david@redhat.com>
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RH-Commit: [1/10] eb774aee79864052e14e706d931e52e7bd1162c8
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Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2168472
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commit 7455ff1aa01564cc175db5b2373e610503ad4411
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Author: Emanuele Giuseppe Esposito <eesposit@redhat.com>
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Date: Tue May 24 13:30:54 2022 -0400
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aio_wait_kick: add missing memory barrier
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It seems that aio_wait_kick always required a memory barrier
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or atomic operation in the caller, but nobody actually
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took care of doing it.
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Let's put the barrier in the function instead, and pair it
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with another one in AIO_WAIT_WHILE. Read aio_wait_kick()
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comment for further explanation.
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Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
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Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
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Message-Id: <20220524173054.12651-1-eesposit@redhat.com>
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Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
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Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
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---
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include/block/aio-wait.h | 2 ++
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util/aio-wait.c | 16 +++++++++++++++-
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2 files changed, 17 insertions(+), 1 deletion(-)
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diff --git a/include/block/aio-wait.h b/include/block/aio-wait.h
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index b39eefb38d..54840f8622 100644
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--- a/include/block/aio-wait.h
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+++ b/include/block/aio-wait.h
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@@ -81,6 +81,8 @@ extern AioWait global_aio_wait;
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AioContext *ctx_ = (ctx); \
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/* Increment wait_->num_waiters before evaluating cond. */ \
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qatomic_inc(&wait_->num_waiters); \
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+ /* Paired with smp_mb in aio_wait_kick(). */ \
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+ smp_mb(); \
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if (ctx_ && in_aio_context_home_thread(ctx_)) { \
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while ((cond)) { \
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aio_poll(ctx_, true); \
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diff --git a/util/aio-wait.c b/util/aio-wait.c
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index bdb3d3af22..98c5accd29 100644
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--- a/util/aio-wait.c
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+++ b/util/aio-wait.c
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@@ -35,7 +35,21 @@ static void dummy_bh_cb(void *opaque)
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void aio_wait_kick(void)
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{
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- /* The barrier (or an atomic op) is in the caller. */
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+ /*
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+ * Paired with smp_mb in AIO_WAIT_WHILE. Here we have:
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+ * write(condition);
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+ * aio_wait_kick() {
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+ * smp_mb();
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+ * read(num_waiters);
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+ * }
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+ *
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+ * And in AIO_WAIT_WHILE:
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+ * write(num_waiters);
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+ * smp_mb();
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+ * read(condition);
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+ */
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+ smp_mb();
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+
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if (qatomic_read(&global_aio_wait.num_waiters)) {
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aio_bh_schedule_oneshot(qemu_get_aio_context(), dummy_bh_cb, NULL);
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}
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--
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2.37.3
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