From 2f03293910f3ac559f37d45c95325ae29638003a Mon Sep 17 00:00:00 2001 From: Emanuele Giuseppe Esposito 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 RH-MergeRequest: 263: qatomic: add smp_mb__before/after_rmw() RH-Bugzilla: 2168472 RH-Acked-by: Cornelia Huck RH-Acked-by: Eric Auger RH-Acked-by: Paolo Bonzini RH-Acked-by: David Hildenbrand RH-Commit: [7/10] 9cf1b6d3b0dd154489e75ad54a3000ea58983960 Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2168472 commit e3a3b6ec8169eab2feb241b4982585001512cd55 Author: Paolo Bonzini 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 Reviewed-by: David Hildenbrand Signed-off-by: Paolo Bonzini Signed-off-by: Emanuele Giuseppe Esposito --- 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