lkml.org 
[lkml]   [2006]   [Oct]   [31]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] splice : two smp_mb() can be omitted
Eric Dumazet wrote:

>On Tuesday 31 October 2006 10:40, Nick Piggin wrote:
>
>
>>Uh, there is nothing that says mutex_unlock or any unlock
>>functions contain an implicit smp_mb(). What is given is that the
>>lock and unlock obey aquire and release memory ordering,
>>respectively.
>>
>>a = x;
>>xxx_unlock
>>b = y;
>>
>>In this situation, the load of y can be executed before that of x.
>>And some architectures will even do so (i386 can, because the
>>unlock is an unprefixed store; ia64 can, because it uses a release
>>barrier in the unlock).
>>
>
>Hum... it seems your mutex_unlock() i386/x86_64 copy is not same as mine :)
>

OK, replace xxx with mutex, and what I've said still holds true for ia64.

>Maybe we could document the fact that mutex_{lock|unlock}() has or has not an
>implicit smp_mb().
>

It does not, none of the unlock functions ever have.

>If not, delete smp_mb() calls from include/asm-generic/mutex-dec.h
>

They should be deleted (and from mutex-xchg). NOT because there is no
need for
a memory barrier, but because the atomic_alter_value_and_return_something
functions always provide a barrier before and after the operation, as per
Documentation/atomic_ops.txt

Again, lock / unlock operations require acquire / release consistency.
This is a
memory ordering operation. It is not equivalent to smp_mb, though.

--

Send instant messages to your online friends http://au.messenger.yahoo.com

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2006-10-31 23:19    [W:0.235 / U:0.568 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site