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From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <1f6310c5-3773-66c5-7f8b-e2f06beb12f8@redhat.com>
Date: Wed, 18 May 2016 16:40:16 +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: 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 18/05/2016 16:10, Sergey Fedorov wrote:
> On 18/05/16 17:05, Paolo Bonzini wrote:
>> 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...

Because atomics cannot create new unconditional writes (and neither can
volatile).

Thanks,

paolo