From 7a9907c65e3e2bbb0c119acdbbeb4381e7f1d902 Mon Sep 17 00:00:00 2001
From: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Date: Thu, 9 Mar 2023 08:24:36 -0500
Subject: [PATCH 09/12] qemu-coroutine-lock: add smp_mb__after_rmw()

RH-Author: Emanuele Giuseppe Esposito <eesposit@redhat.com>
RH-MergeRequest: 158: qatomic: add smp_mb__before/after_rmw()
RH-Bugzilla: 2175660
RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
RH-Acked-by: Cornelia Huck <cohuck@redhat.com>
RH-Acked-by: David Hildenbrand <david@redhat.com>
RH-Acked-by: Eric Auger <eric.auger@redhat.com>
RH-Commit: [6/9] 4b1723b1ad670ec4c85240390b4fc15ff361154f (eesposit/qemu-kvm)

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2175660

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 45c6b57374..c5897bd963 100644
--- a/util/qemu-coroutine-lock.c
+++ b/util/qemu-coroutine-lock.c
@@ -202,10 +202,16 @@ 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) {
@@ -304,6 +310,7 @@ 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