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

[Kevin Wolf]

Am 05.10.2016 um 16:25 hat Paolo Bonzini geschrieben:
> 
> 
> On 05/10/2016 16:20, Kevin Wolf wrote:
> > Am 05.10.2016 um 15:55 hat Paolo Bonzini geschrieben:
> >> 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?
> > 
> > If the comment is wrong, either post a v2 of this patch or just reply
> > with a new version of the comment and I'll squash it in. Your choice, I
> > don't mind either way.
> 
> Just removing those three lines would be okay.

Ok, done.

Kevin