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Re: [Qemu-devel] [PATCH 1/2] async: add aio_bh_schedule_oneshot
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH 1/2] async: add aio_bh_schedule_oneshot |
Date: |
Wed, 5 Oct 2016 16:20:10 +0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
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
Re: [Qemu-devel] [PATCH 0/2] block: introduce and use aio_bh_schedule_oneshot, Kevin Wolf, 2016/10/05