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[PATCH 7/8] qemu-coroutine-lock: add smp_mb__after_rmw()
From: |
Paolo Bonzini |
Subject: |
[PATCH 7/8] qemu-coroutine-lock: add smp_mb__after_rmw() |
Date: |
Fri, 3 Mar 2023 18:19:38 +0100 |
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.
Signed-off-by: Paolo Bonzini <pbonzini@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 58f3f771817b..84a50a9e9117 100644
--- a/util/qemu-coroutine-lock.c
+++ b/util/qemu-coroutine-lock.c
@@ -201,10 +201,16 @@ 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) {
@@ -303,6 +309,7 @@ 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
--
2.39.1
- Re: [PATCH 4/8] edu: add smp_mb__after_rmw(), (continued)
[PATCH 5/8] util/async: add smp_mb__after_rmw() around BH enqueue/dequeue, Paolo Bonzini, 2023/03/03
[PATCH 6/8] aio-wait: switch to smp_mb__after_rmw(), Paolo Bonzini, 2023/03/03
[PATCH 7/8] qemu-coroutine-lock: add smp_mb__after_rmw(),
Paolo Bonzini <=
[PATCH 8/8] physmem: add missing memory barrier, Paolo Bonzini, 2023/03/03
Re: [PATCH 0/8] Fix missing memory barriers on ARM, David Hildenbrand, 2023/03/06