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From: Richard Henderson <rth@twiddle.net>
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Date: Fri, 15 Apr 2016 09:22:49 -0700
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Subject: Re: [Qemu-devel] [RFC v1 05/12] atomic: introduce cmpxchg_bool
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To: =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>, mttcg@greensocs.com, fred.konrad@greensocs.com, a.rigo@virtualopensystems.com, serge.fdrv@gmail.com, cota@braap.org
Cc: qemu-devel@nongnu.org, mark.burton@greensocs.com, pbonzini@redhat.com, jan.kiszka@siemens.com, peter.maydell@linaro.org, claudio.fontana@huawei.com

On 04/15/2016 07:23 AM, Alex Bennée wrote:
> +#define atomic_bool_cmpxchg(ptr, old, new)                              \
> +    ({                                                                  \
> +    typeof(*ptr) _old = (old), _new = (new);                            \
> +    bool r;                                                             \
> +    r = __atomic_compare_exchange(ptr, &_old, &_new, false,             \
> +                                  __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);  \
> +    r;                                                                  \
> +    })

How are you thinking this will be used?  If a loop like

  do {
    old = atomic_read (ptr);
    new = f(old);
  } while (!atomic_bool_cmpxchg(ptr, old, new));

then it's usually helpful to use a weak compare_exchange (s/false/true/ above).
 This will produce one loop for ll/sc architectures instead of two.


r~