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[PATCH v2 6/9] qemu-coroutine-lock: add smp_mb__after_rmw()
From: |
Paolo Bonzini |
Subject: |
[PATCH v2 6/9] qemu-coroutine-lock: add smp_mb__after_rmw() |
Date: |
Mon, 6 Mar 2023 23:33:03 +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.
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
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
- [PATCH v2 0/9] Fix missing memory barriers on ARM, Paolo Bonzini, 2023/03/06
- [PATCH v2 1/9] qatomic: add smp_mb__before/after_rmw(), Paolo Bonzini, 2023/03/06
- [PATCH v2 2/9] qemu-thread-posix: cleanup, fix, document QemuEvent, Paolo Bonzini, 2023/03/06
- [PATCH v2 3/9] qemu-thread-win32: cleanup, fix, document QemuEvent, Paolo Bonzini, 2023/03/06
- [PATCH v2 4/9] edu: add smp_mb__after_rmw(), Paolo Bonzini, 2023/03/06
- [PATCH v2 5/9] aio-wait: switch to smp_mb__after_rmw(), Paolo Bonzini, 2023/03/06
- [PATCH v2 6/9] qemu-coroutine-lock: add smp_mb__after_rmw(),
Paolo Bonzini <=
- [PATCH v2 7/9] physmem: add missing memory barrier, Paolo Bonzini, 2023/03/06
- [PATCH v2 8/9] async: update documentation of the memory barriers, Paolo Bonzini, 2023/03/06
- Re: [PATCH v2 8/9] async: update documentation of the memory barriers, Stefan Hajnoczi, 2023/03/06
- Re: [PATCH v2 8/9] async: update documentation of the memory barriers, Richard Henderson, 2023/03/06
- Re: [PATCH v2 8/9] async: update documentation of the memory barriers, Paolo Bonzini, 2023/03/07
- Re: [PATCH v2 8/9] async: update documentation of the memory barriers, Richard Henderson, 2023/03/07
- Re: [PATCH v2 8/9] async: update documentation of the memory barriers, Paolo Bonzini, 2023/03/07
- Re: [PATCH v2 8/9] async: update documentation of the memory barriers, Richard Henderson, 2023/03/07
- Re: [PATCH v2 8/9] async: update documentation of the memory barriers, Paolo Bonzini, 2023/03/08
- Re: [PATCH v2 8/9] async: update documentation of the memory barriers, Richard Henderson, 2023/03/08