diff options
| -rw-r--r-- | async.c | 28 |
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); + } } |