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Re: [Qemu-devel] [PATCH v1 3/5] include/qemu/atomic.h: default to __atom

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH v1 3/5] include/qemu/atomic.h: default to __atomic functions
Date: Mon, 4 Apr 2016 22:15:13 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

On 04/04/2016 19:03, Paolo Bonzini wrote:
> I've looked at private email from that time and I was pointed to this
> sentence in GCC's manual, which says the opposite:
>     "Note that in the C++11 memory model, fences (e.g.,
>     ‘__atomic_thread_fence’) take effect in combination with other
>     atomic operations on specific memory locations (e.g., atomic loads);
>     operations on specific memory locations do not necessarily affect
>     other operations in the same way."

And GCC is right, based on N3291 paragraph 29.8 ("Fences"):

A release fence A synchronizes with an acquire fence B if there exist
atomic operations X and Y, both operating on some atomic object M, such
that A is sequenced before X, X modifies M, Y is sequenced before B, and
Y reads the value written by X or a value written by any side effect in
the hypothetical release sequence X would head if it were a release

It only mentions atomic operations, not plain loads and stores.


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