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    SubjectRe: Unnecessary barrier in sync_page()?
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    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


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