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From: Sergey Fedorov <serge.fdrv@gmail.com>
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Date: Wed, 18 May 2016 17:47:25 +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: "Emilio G. Cota" <cota@braap.org>
Cc: Richard Henderson <rth@twiddle.net>, QEMU Developers <qemu-devel@nongnu.org>, MTTCG Devel <mttcg@greensocs.com>, =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>, Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>

On 18/05/16 17:18, Sergey Fedorov wrote:
> On 18/05/16 03:28, Emilio G. Cota wrote:
>> On Tue, May 17, 2016 at 23:20:11 +0300, Sergey Fedorov wrote:
>>> On 17/05/16 23:04, Emilio G. Cota wrote:
>> (snip)
>>>> +/*
>>>> + * We might we tempted to use __atomic_test_and_set with __ATOMIC_ACQUIRE;
>>>> + * however, the documentation explicitly says that we should only pass
>>>> + * a boolean to it, so we use __sync_lock_test_and_set, which doesn't
>>>> + * have this limitation, and is documented to have acquire semantics.
>>>> + */
>>>> +#define atomic_test_and_set_acquire(ptr) __sync_lock_test_and_set(ptr, true)
>>> So you are going to stick to *legacy* built-ins?
>> Why not? AFAIK the reason to avoid __sync primitives is that in most cases
>> they include barriers that callers might not necessarily need; __atomic's
>> allow for finer tuning, which is in general a good thing. However,
>> __sync_test_and_set has the exact semantics we need, without the limitations
>> documented for __atomic_test_and_set; so why not use it?
> So it should be okay as long as the legacy build-ins are supported.

However, there's also __atomic_compare_exchange_n(). Could it be the choice?

Kind regards,
Sergey