There is no implicit memory barrier in qatomic_fetch_or() and
atomic_fetch_and() on ARM systems. Add an explicit
smp_mb__after_rmw() to match the intended semantics.
Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
---
util/async.c | 13 ++++++++-----
1 file changed, 8 insertions(+), 5 deletions(-)
diff --git a/util/async.c b/util/async.c
index 0657b7539777..6129f2c991cb 100644
--- a/util/async.c
+++ b/util/async.c
@@ -74,13 +74,15 @@ static void aio_bh_enqueue(QEMUBH *bh, unsigned new_flags)
unsigned old_flags;
/*
- * The memory barrier implicit in qatomic_fetch_or makes sure that:
- * 1. idle & any writes needed by the callback are done before the
- * locations are read in the aio_bh_poll.
+ * The memory barrier makes sure that:
+ * 1. any writes needed by the callback are visible from the callback
+ * after aio_bh_dequeue() returns bh.
* 2. ctx is loaded before the callback has a chance to execute and bh
* could be freed.
*/
old_flags = qatomic_fetch_or(&bh->flags, BH_PENDING | new_flags);
+ smp_mb__after_rmw();
+
if (!(old_flags & BH_PENDING)) {
QSLIST_INSERT_HEAD_ATOMIC(&ctx->bh_list, bh, next);
}
@@ -107,14 +109,15 @@ static QEMUBH *aio_bh_dequeue(BHList *head, unsigned
*flags)
QSLIST_REMOVE_HEAD(head, next);
/*
- * The qatomic_and is paired with aio_bh_enqueue(). The implicit memory
- * barrier ensures that the callback sees all writes done by the scheduling
+ * The memory barrier is paired with aio_bh_enqueue() and it
+ * ensures that the callback sees all writes done by the scheduling
* thread. It also ensures that the scheduling thread sees the cleared
* flag before bh->cb has run, and thus will call aio_notify again if
* necessary.
*/
*flags = qatomic_fetch_and(&bh->flags,
~(BH_PENDING | BH_SCHEDULED | BH_IDLE));
+ smp_mb__after_rmw();
return bh;
}
--
2.39.1
