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

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Date:	Tue, 31 Oct 2006 20:40:21 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Eric Dumazet <dada1@...mosbay.com>
CC:	Andrew Morton <akpm@...l.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [PATCH] splice : two smp_mb() can be omitted

Eric Dumazet wrote:
> This patch deletes two calls to smp_mb() that were done after 
> mutex_unlock() that contains an implicit memory barrier.

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).

Whenever you rely on orderings of things *outside* locks (even
partially outside), you do need to be very careful about barriers
and can't rely on locks to do the right thing for you.

> 
> 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.

It *may* be the case that these can be removed, but not by virtue
of the fact that the smp_mb is redundant.

> 
> Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
> 
> 
> ------------------------------------------------------------------------
> 
> --- linux/fs/splice.c	2006-10-31 07:49:52.000000000 +0100
> +++ linux-ed/fs/splice.c	2006-10-31 08:04:58.000000000 +0100
> @@ -248,7 +248,6 @@
>  		mutex_unlock(&pipe->inode->i_mutex);
>  
>  	if (do_wakeup) {
> -		smp_mb();
>  		if (waitqueue_active(&pipe->wait))
>  			wake_up_interruptible(&pipe->wait);
>  		kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
> @@ -1518,7 +1517,6 @@
>  	 * If we put data in the output pipe, wakeup any potential readers.
>  	 */
>  	if (ret > 0) {
> -		smp_mb();
>  		if (waitqueue_active(&opipe->wait))
>  			wake_up_interruptible(&opipe->wait);
>  		kill_fasync(&opipe->fasync_readers, SIGIO, POLL_IN);


-- 
SUSE Labs, Novell Inc.
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