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

[Kevin Wolf]

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.

Kevin