Re: [Qemu-block] [Qemu-devel] [PATCH v3] util/async: use atomic_mb_set in qemu_bh_cancel

[Pavel Butsykin]

On 08.11.2017 17:15, Paolo Bonzini wrote:
> On 08/11/2017 15:10, Sergio Lopez wrote:
> > > I'm not quite sure that the pre-fetched is involved in this issue,
> > > because pre-fetch reading a certain addresses should be invalidated by
> > > write on another core to the same addresses. In our case write
> > > req->state = THREAD_DONE should invalidate read req->state == THREAD_DONE.
> > > I am inclined to think that there is a memory-reordering read with
> > > write. It's a very real case for x86 and I don't see the reasons which
> > > can prevent it:
> > Yes, you're right. This is actually a memory reordering issue. I'm
> > going to rewrite that paragraph.
>
> Well, memory reordering _is_ caused by speculative prefetching, delayed
> cache invalidation (store buffers), and so on.

what do you mean?

If we are speaking about x86, then a write on another core
(like req->state = THREAD_DONE in this issue) should invalidate
prefetch read(req->state = THREAD_DONE) and this is prevented in
hardware. The prefetch is locked to the L1, when another cpu
invalidates the cache lines, the prefetch is invalidated also
(As far as I understand it).

> But it's probably better indeed to replace "pre-fetched" with
> "outdated".  Whoever commits the patch can do the substitution (I can too).
>
> Paolo