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Date: Wed, 7 Jul 2004 11:29:53 -0700
From: Andrew Morton <akpm@osdl.org>
To: Andrea Arcangeli <andrea@suse.de>
Cc: marcelo.tosatti@cyclades.com, mason@suse.com, linux-kernel@vger.kernel.org
Subject: Re: Unnecessary barrier in sync_page()?
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Andrea Arcangeli <andrea@suse.de> wrote:
>
> however the smp_mb() isn't needed in sync_page, simply because it's
>  perfectly ok if we start running sync_page before reading pagelocked.
>  All we care about is to run sync_page _before_ io_schedule() and that we
>  read PageLocked _after_ prepare_to_wait_exclusive.
> 
>  So the locking in between PageLocked and sync_page is _absolutely_
>  worthless and the smp_mb() can go away.

IIRC, Chris added that barrier (and several similar) for the reads in
page_mapping().