linux-kernel - Re: [PATCH 19/19] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock

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Date:	Wed, 14 May 2014 21:29:45 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Vlastimil Babka <vbabka@...e.cz>, Jan Kara <jack@...e.cz>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH 19/19] mm: filemap: Avoid unnecessary barries and
	waitqueue lookups in unlock_page fastpath

On 05/14, Oleg Nesterov wrote:
>
> The subsequent discussion was "off-topic", and it seems that the patch
> itself needs a bit more discussion,
>
> On 05/13, Peter Zijlstra wrote:
> >
> > On Tue, May 13, 2014 at 01:53:13PM +0100, Mel Gorman wrote:
> > > On Tue, May 13, 2014 at 10:45:50AM +0100, Mel Gorman wrote:
> > > >  void unlock_page(struct page *page)
> > > >  {
> > > > +	wait_queue_head_t *wqh = clear_page_waiters(page);
> > > > +
> > > >  	VM_BUG_ON_PAGE(!PageLocked(page), page);
> > > > +
> > > > +	/*
> > > > +	 * No additional barrier needed due to clear_bit_unlock barriering all updates
> > > > +	 * before waking waiters
> > > > +	 */
> > > >  	clear_bit_unlock(PG_locked, &page->flags);
> > > > -	smp_mb__after_clear_bit();
> > > > -	wake_up_page(page, PG_locked);
> > >
> > > This is wrong.
>
> Yes,
>
> > > The smp_mb__after_clear_bit() is still required to ensure
> > > that the cleared bit is visible before the wakeup on all architectures.
>
> But note that "the cleared bit is visible before the wakeup" is confusing.
> I mean, we do not need mb() before __wake_up(). We need it only because
> __wake_up_bit() checks waitqueue_active().

OOPS. Sorry Mel, I wrote this looking at the chunk above.  But when I found
the whole patch http://marc.info/?l=linux-mm&m=139997442008267 I see that
it removes waitqueue_active(), so this can be correct. I do not really know,
so far I can't say I fully understand this PageWaiters() trick.

Hmm. But at least prepare_to_wait_exclusive() doesn't look right ;)

If nothing else, this needs abort_exclusive_wait() if killed. And while
"exclusive" is probably fine for __lock_page.*(), I am not sure that
__wait_on_page_locked_*() should be exclusive.

Oleg.

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