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From: Sergey Fedorov <serge.fdrv@gmail.com>
Message-ID: <573C77CB.1010508@gmail.com>
Date: Wed, 18 May 2016 17:10:19 +0300
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Subject: Re: [Qemu-devel] [PATCH v5 07/18] qemu-thread: add simple
 test-and-set spinlock
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To: Paolo Bonzini <pbonzini@redhat.com>, "Emilio G. Cota" <cota@braap.org>
Cc: QEMU Developers <qemu-devel@nongnu.org>, MTTCG Devel <mttcg@greensocs.com>, =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Richard Henderson <rth@twiddle.net>

On 18/05/16 17:05, Paolo Bonzini wrote:
>
> On 18/05/2016 15:59, Sergey Fedorov wrote:
>> But actually (cf include/qemu/atomic.h) we can have:
>>
>>     #define atomic_read(ptr)                              \
>>         ({                                                \
>>         QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
>>         typeof(*ptr) _val;                                \
>>          __atomic_load(ptr, &_val, __ATOMIC_RELAXED);     \
>>         _val;                                             \
>>         })
>>
>>
>> I can't find anywhere if this __atomic_load() has volatile/compiler
>> barrier semantics...
> The standard says "you can have data races on atomic loads", that is
> very close to compiler barrier semantics but indeed atomics.txt should
> be updated to explain the C11 memory model in not-so-formal terms.
>
> For example this:
>
>   atomic_set(&x, 1);
>   atomic_set(&y, 1);
>   atomic_set(&x, 2);
>   atomic_set(&y, 2);
>
> could become
>
>   atomic_set(&x, 2);
>   atomic_set(&y, 2);
>
> with C11 atomics but not with volatile.  However this:
>
>   if (atomic_read(&x) != 1) {
>     atomic_set(&x, 1);
>   }
>
> couldn't become an unconditional
>
>   atomic_set(&x, 1);

Sorry, I can't figure out why it couldn't...

Kind regards,
Sergey