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Re: [PATCH] Fix atomic_full_barrier on x86 and x86_64.

From: Mike Frysinger <vapier at gentoo dot org>
To: Torvald Riegel <triegel at redhat dot com>
Cc: "Joseph S. Myers" <joseph at codesourcery dot com>, Rich Felker <dalias at libc dot org>, GLIBC Devel <libc-alpha at sourceware dot org>
Date: Thu, 5 Mar 2015 13:19:09 -0500
Subject: Re: [PATCH] Fix atomic_full_barrier on x86 and x86_64.
Authentication-results: sourceware.org; auth=none
References: <1414606736 dot 10085 dot 1 dot camel at triegel dot csb> <20141029203158 dot GU22465 at brightrain dot aerifal dot cx> <1414616100 dot 10085 dot 11 dot camel at triegel dot csb> <Pine dot LNX dot 4 dot 64 dot 1410292151470 dot 15119 at digraph dot polyomino dot org dot uk> <1414621937 dot 10085 dot 67 dot camel at triegel dot csb> <Pine dot LNX dot 4 dot 64 dot 1410292236350 dot 15119 at digraph dot polyomino dot org dot uk> <1416912710 dot 1771 dot 176 dot camel at triegel dot csb>

On 25 Nov 2014 11:51, Torvald Riegel wrote:
> --- a/sysdeps/i386/i486/bits/atomic.h
> +++ b/sysdeps/i386/i486/bits/atomic.h
> @@ -535,3 +535,12 @@ typedef uintmax_t uatomic_max_t;
>  #define atomic_or(mem, mask) __arch_or_body (LOCK_PREFIX, mem, mask)
>  
>  #define catomic_or(mem, mask) __arch_or_body (__arch_cprefix, mem, mask)
> +
> +#ifdef __SSE2__
> +# define atomic_full_barrier() __asm ("mfence" ::: "memory")
> +#else
> +# define atomic_full_barrier() \
> +    __asm __volatile (LOCK_PREFIX "orl $0, (%%esp)" ::: "memory")
> +#endif

so this will kick in only when glibc itself is compiled with -msse2/etc...
support.  then again, these barriers only get used by glibc internal code,
so i guess this is the best answer.  plus it only impacts x86, and it's not
like anyone really cares about that anymore ;).

lgtm
-mike

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