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-rw-r--r--async.c28
1 files changed, 12 insertions, 16 deletions
diff --git a/async.c b/async.c
index 2be88cc9e9..2b51e87679 100644
--- a/async.c
+++ b/async.c
@@ -72,12 +72,13 @@ int aio_bh_poll(AioContext *ctx)
/* Make sure that fetching bh happens before accessing its members */
smp_read_barrier_depends();
next = bh->next;
- if (!bh->deleted && bh->scheduled) {
- bh->scheduled = 0;
- /* Paired with write barrier in bh schedule to ensure reading for
- * idle & callbacks coming after bh's scheduling.
- */
- smp_rmb();
+ /* The atomic_xchg is paired with the one in qemu_bh_schedule. The
+ * implicit memory barrier ensures that the callback sees all writes
+ * done by the scheduling thread. It also ensures that the scheduling
+ * thread sees the zero before bh->cb has run, and thus will call
+ * aio_notify again if necessary.
+ */
+ if (!bh->deleted && atomic_xchg(&bh->scheduled, 0)) {
if (!bh->idle)
ret = 1;
bh->idle = 0;
@@ -108,33 +109,28 @@ int aio_bh_poll(AioContext *ctx)
void qemu_bh_schedule_idle(QEMUBH *bh)
{
- if (bh->scheduled)
- return;
bh->idle = 1;
/* Make sure that idle & any writes needed by the callback are done
* before the locations are read in the aio_bh_poll.
*/
- smp_wmb();
- bh->scheduled = 1;
+ atomic_mb_set(&bh->scheduled, 1);
}
void qemu_bh_schedule(QEMUBH *bh)
{
AioContext *ctx;
- if (bh->scheduled)
- return;
ctx = bh->ctx;
bh->idle = 0;
- /* Make sure that:
+ /* The memory barrier implicit in atomic_xchg makes sure that:
* 1. idle & any writes needed by the callback are done before the
* locations are read in the aio_bh_poll.
* 2. ctx is loaded before scheduled is set and the callback has a chance
* to execute.
*/
- smp_mb();
- bh->scheduled = 1;
- aio_notify(ctx);
+ if (atomic_xchg(&bh->scheduled, 1) == 0) {
+ aio_notify(ctx);
+ }
}