Re: 2.6.22 -mm merge plans -- vm bugfixes

[Nick Piggin]

Hugh Dickins wrote:
> 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).

Yeah, I was getting too clever for my own boots :)

I think the patch has merit though. Unfortunate that it uses another page
flag, however it seemed to have quite a bit speedup on unlock_page (probably
from both the barriers and an extra random cacheline load (from the hash)).

I guess it has to get good results from more benchmarks...


> > 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.

That barrier was one too many :)

However I believe the fastpath barrier can go away because the PG_locked
operation is depending on the same cacheline as PG_waiters.

--
SUSE Labs, Novell Inc.
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