linux-kernel - Re: [PATCH] splice : two smp

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Message-id: <4546FE39.8000201@cosmosbay.com>
Date:	Tue, 31 Oct 2006 08:41:45 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Andrew Morton <akpm@...l.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] splice : two smp_mb() can be omitted

Jens Axboe a écrit :
> On Tue, Oct 31 2006, Eric Dumazet wrote:
>> This patch deletes two calls to smp_mb() that were done after 
>> mutex_unlock() that contains an implicit memory barrier.
>>
>> The first one in splice_to_pipe(), where 'do_wakeup' is set to true only if 
>> pipe->inode is set (and in this case the
>> if (pipe->inode)
>>    mutex_unlock(&pipe->inode->i_mutex);
>> is done too)
>>
>> The second one in link_pipe(), following inode_double_unlock() that 
>> contains calls to mutex_unlock() too.
> 
> NAK on that patch, the smp_mb() follows the waitqueue_active(). If you
> later change the code and move the locks or whatnot, you have lost that
> connection.
> 
> If you change the patch to insert a comment, then it may be more
> applicable.
> 

Hum... I read fs/pipe.c and see no smp_mb() there, but I suspect same 
semantics are/were used.

Should we add comments on fs/pipe.c too ?

Eric
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