linux-kernel - Re: 2.6.22 -mm merge plans -- vm bugfixes

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Thu, 03 May 2007 23:08:32 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Hugh Dickins <hugh@...itas.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrea Arcangeli <andrea@...e.de>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: 2.6.22 -mm merge plans -- vm bugfixes

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 from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/