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From: Sergey Fedorov <serge.fdrv@gmail.com>
Message-ID: <5744B50B.5040001@gmail.com>
Date: Tue, 24 May 2016 23:09:47 +0300
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Subject: Re: [Qemu-devel] [PATCH v2 2/3] atomics: emit an
 smp_read_barrier_depends() barrier only for Sparc and Thread Sanitizer
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To: "Emilio G. Cota" <cota@braap.org>, QEMU Developers <qemu-devel@nongnu.org>, MTTCG Devel <mttcg@listserver.greensocs.com>
Cc: =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>, Paolo Bonzini <pbonzini@redhat.com>, Richard Henderson <rth@twiddle.net>

On 24/05/16 23:06, Emilio G. Cota wrote:
> For correctness, smp_read_barrier_depends() is only required to
> emit a barrier on Sparc hosts. However, we are currently emitting
> a consume fence unconditionally.
>
> Fix it by keeping the consume fence if we're compiling with Thread
> Sanitizer, since this might help prevent false warnings. Otherwise,
> only emit the barrier for Sparc hosts. Note that we still guarantee
> that smp_read_barrier_depends() is a compiler barrier.

s/Sparc/Alpha/?

Regards,
Sergey

>
> Signed-off-by: Emilio G. Cota <cota@braap.org>
> ---
>  include/qemu/atomic.h | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
> index 5bc4d6c..4a4f2fb 100644
> --- a/include/qemu/atomic.h
> +++ b/include/qemu/atomic.h
> @@ -36,7 +36,14 @@
>  #define smp_wmb()   ({ barrier(); __atomic_thread_fence(__ATOMIC_RELEASE); barrier(); })
>  #define smp_rmb()   ({ barrier(); __atomic_thread_fence(__ATOMIC_ACQUIRE); barrier(); })
>  
> +#if defined(__SANITIZE_THREAD__)
>  #define smp_read_barrier_depends() ({ barrier(); __atomic_thread_fence(__ATOMIC_CONSUME); barrier(); })
> +#elsif defined(__alpha__)
> +#define smp_read_barrier_depends()   asm volatile("mb":::"memory")
> +#else
> +#define smp_read_barrier_depends()   barrier()
> +#endif
> +
>  
>  /* Weak atomic operations prevent the compiler moving other
>   * loads/stores past the atomic operation load/store. However there is