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Date: Mon, 16 Jan 2012 15:53:12 +0800
From: David Xu <listlog2011@gmail.com>
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To: Lawrence Stewart <lstewart@freebsd.org>
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Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org,
	src-committers@freebsd.org, David Xu <davidxu@freebsd.org>
Subject: Re: svn commit: r230201 - head/lib/libc/gen
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On 2012/1/16 15:39, Lawrence Stewart wrote:
> On 01/16/12 17:15, David Xu wrote:
>> Author: davidxu
>> Date: Mon Jan 16 06:15:14 2012
>> New Revision: 230201
>> URL: http://svn.freebsd.org/changeset/base/230201
>>
>> Log:
>>    Insert read memory barriers.
>>
>> Modified:
>>    head/lib/libc/gen/sem.c
>>    head/lib/libc/gen/sem_new.c
>
> Could you please provide a bit more information about why these are 
> necessary and why they weren't there before (or how you figured out 
> that there was a problem without them)? I learn a lot by reading 
> commit mail, but only when the log message and any added code comments 
> explain the "what" *and* the "why".
>
> Cheers,
> Lawrence
>
I know this is rather obscure and diffcult to understand, the problem is 
here
we have two variables m, n, and two threads A, B.
the A wants to set m, than looks n, while B is in reverse order,
it should be in strict order, but since atomic.h does not have a full 
barrier
atomic operation interface, I intend to add a rmb() here.
though X86 may not need this, but other arches may not work in this way.

Regards,
David Xu