Re: [PATCH] splice : two smp_mb() can be omitted

[Eric Dumazet <dada1@cosmosbay.com>]

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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