On Thu, 3 May 2007, Nick Piggin wrote:
> >>@@ -568,6 +570,11 @@ __lock_page (diff -p would tell us!)
> > > {
> > > DEFINE_WAIT_BIT(wait, &page->flags, PG_locked);
> > >
> > >+ set_bit(PG_waiters, &page->flags);
> > >+ if (unlikely(!TestSetPageLocked(page))) {
> >
> > What happens if another cpu is coming through __lock_page at the
> > same time, did its set_bit, now finds PageLocked, and so proceeds
> > to the __wait_on_bit_lock? But this cpu now clears PG_waiters,
> > so this task's unlock_page won't wake the other?
>
> You're right, we can't clear the bit here. Doubt it mattered much anyway?
Ah yes, that's a good easy answer. In fact, just remove this whole
test and block (we already tried TestSetPageLocked outside just a
short while ago, so this repeat won't often save anything).
>
> BTW. I also forgot an smp_mb__after_clear_bit() before the wake_up_page
> above... that barrier is in the slow path as well though, so it shouldn't
> matter either.
I vaguely wondered how such barriers had managed to dissolve away,
but cranking my brain up to think about barriers takes far too long.
> > >+ clear_bit(PG_waiters, &page->flags);
> > >+ return;
> > >+ }
> > > __wait_on_bit_lock(page_waitqueue(page), &wait, sync_page,
> > > TASK_UNINTERRUPTIBLE);
> >> }
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