[...] >>> >>> 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. > > This is a separate patch anyway... and a long discussion to have before > too. :) > > Let's concentrate on one thing at a time. > Yes, will do like this.
Regards, Pingfan > Paolo > >> 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); > > This is expensive. > >> + 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 >