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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Wed, 18 May 2016 16:59:19 +0200
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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: Sergey Fedorov <serge.fdrv@gmail.com>, "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>, Peter Crosthwaite <crosthwaite.peter@gmail.com>



On 18/05/2016 16:47, Sergey Fedorov wrote:
>>> >> 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?

cmpxchg is not TAS.  I don't see any reason not to use
__sync_test_and_set, the only sensible alternative is to ignore the
standard and use __atomic_test_and_set on int.

Paolo