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

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Date:	Tue, 31 Oct 2006 08:32:12 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Eric Dumazet <dada1@...mosbay.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

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.

-- 
Jens Axboe

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