Re: [Qemu-devel] [PATCH 10/16] block: explicitly acquire aiocontext in timers that need it

[Stefan Hajnoczi]

On Wed, Jan 18, 2017 at 05:44:24PM +0100, Paolo Bonzini wrote:
> On 18/01/2017 16:43, Stefan Hajnoczi wrote:
> > On Fri, Jan 13, 2017 at 02:17:25PM +0100, Paolo Bonzini wrote:
> >> diff --git a/block/null.c b/block/null.c
> >> index b300390..356209a 100644
> >> --- a/block/null.c
> >> +++ b/block/null.c
> >> @@ -141,7 +141,11 @@ static void null_bh_cb(void *opaque)
> >>  static void null_timer_cb(void *opaque)
> >>  {
> >>      NullAIOCB *acb = opaque;
> >> +    AioContext *ctx = bdrv_get_aio_context(acb->common.bs);
> >> +
> >> +    aio_context_acquire(ctx);
> >>      acb->common.cb(acb->common.opaque, 0);
> >> +    aio_context_release(ctx);
> >>      timer_deinit(&acb->timer);
> >>      qemu_aio_unref(acb);
> > 
> > Is qemu_aio_unref() thread-safe?
> 
> qemu_aio_ref()/qemu_aio_unref() is only used by bdrv_aio_cancel, which
> in turn is not used by dataplane.  So in the multithreaded case
> qemu_aio_unref() is effectively qemu_aio_free().
> 
> Probably needs more documentation, or a different implementation of
> bdrv_aio_cancel (e.g. replacing the reference counting with a
> NotifierList of some kind).  Let me know what you prefer for v2.

Documentation is fine.  I just checked and see that virtio-scsi
dataplane uses blk_aio_cancel_async() so the aio refcount is never
touched - no race.

Stefan

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