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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
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>
This commit is contained in:
Paolo Bonzini 2023-03-03 10:52:59 +01:00
parent b532526a07
commit e3a3b6ec81
1 changed files with 8 additions and 1 deletions
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9
util/qemu-coroutine-lock.c
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@ -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