Re: [PATCH v2] util/async: Add memory barrier to aio_ctx_prepare

[Stefan Hajnoczi]

On Thu, Apr 02, 2020 at 11:59:06AM +0200, Paolo Bonzini wrote:
> On 02/04/20 11:32, Stefan Hajnoczi wrote:
> > Paolo, I'm not sure how to interpret this case according to
> > docs/devel/atomics.txt.  Maybe you can clarify.
> > 
> > atomic_or() is sequentially consistent and I therefore expected it to
> > act as a barrier.  Or does sequential consistency only cover the memory
> > accessed via the sequentially consistent atomics APIs and everything
> > else (like aio_compute_timeout()) can be reordered?
> 
> Yes, that's what I expected too when I wrote that code. :(  But Torvald
> Riegel explained a while ago that seq-cst accesses are actually weaker
> than e.g. the Linux kernel atomics[1].
> 
> The difference basically only matters if you are writing the relatively
> common synchronization pattern
> 
>       write A                         write B
>       smp_mb()                        smp_mb()
>       read B                          read A
>       if not B then sleep             if A then wake up the other side
>       do something
> 
> because you must either use memory barriers as above, or use seq-cst
> writes *and* reads.  You cannot rely on having a memory barrier implicit
> in the writes.
> 
> Paolo
> 
> [1] https://lists.gnu.org/archive/html/qemu-arm/2019-10/msg00051.html

Thanks.  I know you are probably very busy right now, but updating
atomics.txt would be great.

Stefan

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