Re: [Qemu-devel] [PATCH 1/2] async: add aio_bh_schedule_oneshot

[Paolo Bonzini]

On 05/10/2016 15:13, Stefan Hajnoczi wrote:
> > qemu_bh_delete is already clearing bh->scheduled at the same time
> > as it's setting bh->deleted.  Since it's not using any memory
> > barriers, there is no synchronization going on for bh->deleted,
> > and this makes the bh->deleted checks superfluous in aio_compute_timeout,
> > aio_bh_poll and aio_ctx_check.
> 
> Yikes.  On one hand this sounds scary but in practice qemu_bh_delete()
> isn't called from another thread so the next aio_bh_poll() will indeed
> clean it up instead of dispatching a deleted BH.
> 
> Due to the nature of this change I suggest making it in a separate
> patch.

Separate from what?  (Sorry if I'm being dense).

>>
>> + * aio_bh_schedule_oneshot: Allocate a new bottom half structure that will 
>> run
>> + * only once and as soon as possible.
>> + *
>> + * Bottom halves are lightweight callbacks whose invocation is guaranteed
>> + * to be wait-free, thread-safe and signal-safe.  The #QEMUBH structure
>> + * is opaque and must be allocated prior to its use.
> 
> I'm confused.  There is no QEMUBH structure in this function
> prototype.  Is this comment from an earlier version of this function?

No, it's from aio_bh_new.  Of course this one is neither wait-free nor
signal-safe.  Kevin, do you want me to respin?

Paolo