Re: [Qemu-devel] [PATCH v3 2/2] QEMUBH: make AioContext's bh re-entrant

[Paolo Bonzini <pbonzini@redhat.com>]

Il 20/06/2013 11:41, liu ping fan ha scritto:
> On Thu, Jun 20, 2013 at 4:16 PM, Paolo Bonzini <pbonzini@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.

This is a separate patch anyway... and a long discussion to have before
too. :)

Let's concentrate on one thing at a time.

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