qemu-kvm/kvm-qemu-coroutine-lock-add-smp_mb__after_rmw.patch

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2023-04-04 12:34:43 +00:00
From 2f03293910f3ac559f37d45c95325ae29638003a Mon Sep 17 00:00:00 2001
From: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Date: Thu, 9 Mar 2023 08:15:14 -0500
Subject: [PATCH 07/13] qemu-coroutine-lock: add smp_mb__after_rmw()
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: [7/10] 9cf1b6d3b0dd154489e75ad54a3000ea58983960
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2168472
commit e3a3b6ec8169eab2feb241b4982585001512cd55
Author: Paolo Bonzini <pbonzini@redhat.com>
Date: Fri Mar 3 10:52:59 2023 +0100
qemu-coroutine-lock: add smp_mb__after_rmw()
mutex->from_push and mutex->handoff in qemu-coroutine-lock implement
the familiar pattern:
write a write b
smp_mb() smp_mb()
read b read a
The memory barrier is required by the C memory model even after a
SEQ_CST read-modify-write operation such as QSLIST_INSERT_HEAD_ATOMIC.
Add it and avoid the unclear qatomic_mb_read() operation.
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
util/qemu-coroutine-lock.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/util/qemu-coroutine-lock.c b/util/qemu-coroutine-lock.c
index 2669403839..a03ed0e664 100644
--- a/util/qemu-coroutine-lock.c
+++ b/util/qemu-coroutine-lock.c
@@ -206,10 +206,16 @@ static void coroutine_fn qemu_co_mutex_lock_slowpath(AioContext *ctx,
trace_qemu_co_mutex_lock_entry(mutex, self);
push_waiter(mutex, &w);
+ /*
+ * Add waiter before reading mutex->handoff. Pairs with qatomic_mb_set
+ * in qemu_co_mutex_unlock.
+ */
+ smp_mb__after_rmw();
+
/* This is the "Responsibility Hand-Off" protocol; a lock() picks from
* a concurrent unlock() the responsibility of waking somebody up.
*/
- old_handoff = qatomic_mb_read(&mutex->handoff);
+ old_handoff = qatomic_read(&mutex->handoff);
if (old_handoff &&
has_waiters(mutex) &&
qatomic_cmpxchg(&mutex->handoff, old_handoff, 0) == old_handoff) {
@@ -308,6 +314,7 @@ void coroutine_fn qemu_co_mutex_unlock(CoMutex *mutex)
}
our_handoff = mutex->sequence;
+ /* Set handoff before checking for waiters. */
qatomic_mb_set(&mutex->handoff, our_handoff);
if (!has_waiters(mutex)) {
/* The concurrent lock has not added itself yet, so it
--
2.37.3