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From: Sergey Fedorov <serge.fdrv@gmail.com>
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Date: Tue, 17 May 2016 23:35:57 +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: QEMU Developers <qemu-devel@nongnu.org>, MTTCG Devel <mttcg@listserver.greensocs.com>, =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>, Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Richard Henderson <rth@twiddle.net>

On 17/05/16 22:38, Emilio G. Cota wrote:
> On Tue, May 17, 2016 at 20:13:24 +0300, Sergey Fedorov wrote:
>> On 14/05/16 06:34, Emilio G. Cota wrote:
(snip)
>>> +        while (atomic_read(&spin->value)) {
>>> +            cpu_relax();
>>> +        }
>>> +    }
>> Looks like relaxed atomic access can be a subject to various
>> optimisations according to
>> https://gcc.gnu.org/wiki/Atomic/GCCMM/AtomicSync#Relaxed.
> The important thing here is that the read actually happens
> on every iteration; this is achieved with atomic_read().
> Barriers etc. do not matter here because once we exit
> the loop, the try to acquire the lock -- and if we succeed,
> we then emit the right barrier.

I just can't find where it is stated that an expression like
"__atomic_load(ptr, &_val, __ATOMIC_RELAXED)" has a _compiler_ barrier
or volatile access semantic. Hopefully, cpu_relax() serves as a compiler
barrier. If we rely on that, we'd better put a comment about it.

Kind regards,
Sergey

>>> +static inline bool qemu_spin_locked(QemuSpin *spin)
>>> +{
>>> +    return atomic_read_acquire(&spin->value);
>> Why not just atomic_read()?
> I think atomic_read() is better, yes. I'll change it. I went
> with the fence because I wanted to have at least a caller
> of atomic_read_acquire :P
>
> I also hesitated between calling it _locked or _is_locked;
> I used _locked for consistency with qemu_mutex_iothread_locked,
> although I think _is_locked is a bit clearer:
> qemu_spin_locked(foo)
>    is a little too similar to
> qemu_spin_lock(foo).
>
> Thanks,
>
> 		Emilio