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From 7a9907c65e3e2bbb0c119acdbbeb4381e7f1d902 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 09/12] qemu-coroutine-lock: add smp_mb__after_rmw()
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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: [6/9] 4b1723b1ad670ec4c85240390b4fc15ff361154f (eesposit/qemu-kvm)
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Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2175660
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commit e3a3b6ec8169eab2feb241b4982585001512cd55
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Author: Paolo Bonzini <pbonzini@redhat.com>
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Date: Fri Mar 3 10:52:59 2023 +0100
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qemu-coroutine-lock: add smp_mb__after_rmw()
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mutex->from_push and mutex->handoff in qemu-coroutine-lock implement
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the familiar pattern:
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write a write b
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smp_mb() smp_mb()
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read b read a
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The memory barrier is required by the C memory model even after a
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SEQ_CST read-modify-write operation such as QSLIST_INSERT_HEAD_ATOMIC.
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Add it and avoid the unclear qatomic_mb_read() operation.
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Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
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Reviewed-by: David Hildenbrand <david@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/qemu-coroutine-lock.c | 9 ++++++++-
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1 file changed, 8 insertions(+), 1 deletion(-)
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diff --git a/util/qemu-coroutine-lock.c b/util/qemu-coroutine-lock.c
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index 45c6b57374..c5897bd963 100644
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--- a/util/qemu-coroutine-lock.c
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+++ b/util/qemu-coroutine-lock.c
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@@ -202,10 +202,16 @@ static void coroutine_fn qemu_co_mutex_lock_slowpath(AioContext *ctx,
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trace_qemu_co_mutex_lock_entry(mutex, self);
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push_waiter(mutex, &w);
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+ /*
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+ * Add waiter before reading mutex->handoff. Pairs with qatomic_mb_set
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+ * in qemu_co_mutex_unlock.
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+ */
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+ smp_mb__after_rmw();
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+
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/* This is the "Responsibility Hand-Off" protocol; a lock() picks from
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* a concurrent unlock() the responsibility of waking somebody up.
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*/
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- old_handoff = qatomic_mb_read(&mutex->handoff);
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+ old_handoff = qatomic_read(&mutex->handoff);
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if (old_handoff &&
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has_waiters(mutex) &&
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qatomic_cmpxchg(&mutex->handoff, old_handoff, 0) == old_handoff) {
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@@ -304,6 +310,7 @@ void coroutine_fn qemu_co_mutex_unlock(CoMutex *mutex)
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}
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our_handoff = mutex->sequence;
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+ /* Set handoff before checking for waiters. */
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qatomic_mb_set(&mutex->handoff, our_handoff);
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if (!has_waiters(mutex)) {
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/* The concurrent lock has not added itself yet, so it
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--
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2.39.1
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