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Date: Fri, 20 Jan 2012 08:36:33 +0800
From: David Xu <listlog2011@gmail.com>
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To: John Baldwin <jhb@freebsd.org>
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Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org,
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Subject: Re: svn commit: r230201 - head/lib/libc/gen
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On 2012/1/19 23:23, John Baldwin wrote:
> On Thursday, January 19, 2012 12:57:50 am David Xu wrote:
>> rdtsc() may not work on SMP, so I have updated it to use clock_gettime
>> to get total time.
>> http://people.freebsd.org/~davidxu/bench/semaphore2/
>> <http://people.freebsd.org/%7Edavidxu/bench/semaphore2/>
>>
>> Still, lfence is a lot faster than atomic lock.
> http://www.freebsd.org/~jhb/patches/amd64_fence.patch
>
> This the patch I've had for quite a while.  Can you retest with this?  You'll
> probably have to install the updated header in /usr/include as well.
>
The lines in atomic_load_acq() seem not what I want:

+	v = *p;						\
+	__asm __volatile("lfence" ::: "memory");	\

I think they should be swapped ?

+	__asm __volatile("lfence" ::: "memory");	\
+	v = *p;						\

What I need in the semaphore code is read can not pass write in such a special case.