Re: Unnecessary barrier in sync_page()?

[Chris Mason <mason@suse.com>]

On Wed, 2004-07-07 at 14:42, Andrea Arcangeli wrote:
> On Wed, Jul 07, 2004 at 11:29:53AM -0700, Andrew Morton wrote:
> > 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().
> 
> how does it help to know the page is not locked before executing
> page_mapped?

I wasn't worried about the locked bit when I added the barrier, my goal
was to order things with people that set page->mapping to null.

-chris