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

[Pavel Butsykin <pbutsykin@virtuozzo.com>]

On 08.11.2017 17:24, Sergio Lopez wrote:
> On Wed, Nov 8, 2017 at 3:15 PM, Paolo Bonzini <pbonzini@redhat.com> 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.
>>
>> But it's probably better indeed to replace "pre-fetched" with
>> "outdated".  Whoever commits the patch can do the substitution (I can too).
>>
> 
> Alternatively, if we want to explicitly mention the memory barrier, we
> can replace the third paragraph with something like this:
> 
> <snip>
> This was considered to be safe, as the completion function restarts the
> loop just after the call to qemu_bh_cancel. But, as this loop lacks a HW
> memory barrier, the read of req->state may actually happen _before_ the
> call, seeing it still as THREAD_QUEUED, and ending the completion
> function without having processed a pending TPE linked at pool->head:
> </snip>

Yes, that's better. Thank you.

> ---
> Sergio
>