From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4639DED0.2070507@yahoo.com.au> Date: Thu, 03 May 2007 23:08:32 +1000 From: Nick Piggin MIME-Version: 1.0 Subject: Re: 2.6.22 -mm merge plans -- vm bugfixes References: <20070430162007.ad46e153.akpm@linux-foundation.org> <4636FDD7.9080401@yahoo.com.au> <4638009E.3070408@yahoo.com.au> <46393BA7.6030106@yahoo.com.au> <4639D8E8.2090608@yahoo.com.au> In-Reply-To: Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Hugh Dickins Cc: Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrea Arcangeli , Christoph Hellwig List-ID: 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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org