On Thu, Jun 20, 2013 at 4:16 PM, Paolo Bonzini <pbonz...@redhat.com> wrote: > Il 20/06/2013 09:39, Stefan Hajnoczi ha scritto: >> qemu_bh_cancel() and qemu_bh_delete() are not modified by this patch. >> >> It seems that calling them from a thread is a little risky because there >> is no guarantee that the BH is no longer invoked after a thread calls >> these functions. >> >> I think that's worth a comment or do you want them to take the lock so >> they become safe? > > Taking the lock wouldn't help. The invoking loop of aio_bh_poll runs > lockless. I think a comment is better. > > qemu_bh_cancel is inherently not thread-safe, there's not much you can > do about it. > > qemu_bh_delete is safe as long as you wait for the bottom half to stop > before deleting the containing object. Once we have RCU, deletion of > QOM objects will be RCU-protected. Hence, a simple way could be to put > the first part of aio_bh_poll() within rcu_read_lock/unlock. > In fact, I have some idea about this, introduce another member - Object for QEMUBH which will be refereed in cb, then we leave anything to refcnt mechanism. For qemu_bh_cancel(), I do not figure out whether it is important or not to sync with caller.
diff --git a/async.c b/async.c index 4b17eb7..60c35a1 100644 --- a/async.c +++ b/async.c @@ -61,6 +61,7 @@ int aio_bh_poll(AioContext *ctx) { QEMUBH *bh, **bhp, *next; int ret; + int sched; { QEMUBH *bh, **bhp, *next; int ret; + int sched; ctx->walking_bh++; @@ -69,8 +70,10 @@ int aio_bh_poll(AioContext *ctx) /* Make sure fetching bh before accessing its members */ smp_read_barrier_depends(); next = bh->next; - if (!bh->deleted && bh->scheduled) { - bh->scheduled = 0; + sched = 0; + atomic_xchg(&bh->scheduled, sched); + if (!bh->deleted && sched) { + //bh->scheduled = 0; if (!bh->idle) ret = 1; bh->idle = 0; @@ -79,6 +82,9 @@ int aio_bh_poll(AioContext *ctx) */ smp_rmb(); bh->cb(bh->opaque); + if (bh->obj) { + object_unref(bh->obj); + } } } @@ -105,8 +111,12 @@ int aio_bh_poll(AioContext *ctx) void qemu_bh_schedule_idle(QEMUBH *bh) { - if (bh->scheduled) + int sched = 1; + + atomic_xchg( &bh->scheduled, sched); + if (sched) { return; + } /* Make sure any writes that are needed by the callback are done * before the locations are read in the aio_bh_poll. */ @@ -117,25 +127,46 @@ void qemu_bh_schedule_idle(QEMUBH *bh) void qemu_bh_schedule(QEMUBH *bh) { - if (bh->scheduled) + int sched = 1; + + atomic_xchg( &bh->scheduled, sched); + if (sched) { return; + } /* Make sure any writes that are needed by the callback are done * before the locations are read in the aio_bh_poll. */ smp_wmb(); bh->scheduled = 1; + if (bh->obj) { + object_ref(bh->obj); + } bh->idle = 0; aio_notify(bh->ctx); } void qemu_bh_cancel(QEMUBH *bh) { - bh->scheduled = 0; + int sched = 0; + + atomic_xchg( &bh->scheduled, sched); + if (sched) { + if (bh->obj) { + object_ref(bh->obj); + } + } } void qemu_bh_delete(QEMUBH *bh) { - bh->scheduled = 0; + int sched = 0; + + atomic_xchg( &bh->scheduled, sched); + if (sched) { + if (bh->obj) { + object_ref(bh->obj); + } + } bh->deleted = 1; } Regards, Pingfan >> The other thing I'm unclear on is the ->idle assignment followed >> immediately by a ->scheduled assignment. Without memory barriers >> aio_bh_poll() isn't guaranteed to get an ordered view of these updates: >> it may see an idle BH as a regular scheduled BH because ->idle is still >> 0. > > Right. You need to order ->idle writes before ->scheduled writes, and > add memory barriers, or alternatively use two bits in ->scheduled so > that you can assign both atomically. > > Paolo