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    SubjectRe: Unnecessary barrier in sync_page()?
    On Wed, Jul 07, 2004 at 08:42:02PM +0200, 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 expressed it a bit bad, I mean not locked by us)

    I mean the lock on the page can go from under us anytime. the the very
    best you can try to hide a race condition with such smp_mb, but sure not
    fix it.
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