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From: Alex =?utf-8?Q?Benn=C3=A9e?= <alex.bennee@linaro.org>
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Date: Wed, 12 Oct 2016 10:28:13 +0100
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Subject: Re: [Qemu-devel] [PATCH 5/5] atomic: base mb_read/mb_set on
 load-acquire and store-release
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org, cota@braap.org


Paolo Bonzini <pbonzini@redhat.com> writes:

> This introduces load-acquire and store-release operations in QEMU.
> For now, just use them as an implementation detail of atomic_mb_read
> and atomic_mb_set.
>
> Since docs/atomics.txt documents that atomic_mb_read only synchronizes
> with an atomic_mb_set of the same variable, we can use the new implementation
> everywhere instead of seq-cst loads and stores.
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
<snip>

> +/* This is more efficient than a store plus a fence.  */
> +#if defined(__i386__) || defined(__x86_64__) || defined(__s390x__)
> +#define atomic_mb_set(ptr, i)  ((void)atomic_xchg(ptr, i))
> +#endif

Is this working around a compiler issue? Shouldn't it already be using
the best instructions for the constraint?

> +
> +#ifndef atomic_mb_read
> +#define atomic_mb_read(ptr)                             \
> +    atomic_load_acquire(ptr)
> +#endif
> +
> +#ifndef atomic_mb_set
> +#define atomic_mb_set(ptr, i)  do {                     \
> +    atomic_store_release(ptr, i);                       \
> +    smp_mb();                                           \
> +} while(0)
> +#endif
> +
>  #endif /* QEMU_ATOMIC_H */


--
Alex Bennée