linux-kernel - Re: [rfc] optimise unlock

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]

Message-Id: <1178659827.14928.85.camel@localhost.localdomain>
Date:	Wed, 09 May 2007 07:30:27 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [rfc] optimise unlock_page

On Tue, 2007-05-08 at 13:40 +0200, Nick Piggin wrote:
> This patch trades a page flag for a significant improvement in the unlock_page
> fastpath. Various problems in the previous version were spotted by Hugh and
> Ben (and fixed in this one).
> 
> Comments?
> 
> --
> 
> Speed up unlock_page by introducing a new page flag to signal that there are
> page waitqueue waiters for PG_locked. This means a memory barrier and a random
> waitqueue hash cacheline load can be avoided in the fastpath when there is no
> contention.

I'm not 100% familiar with the exclusive vs. non exclusive wait thingy
but wake_up_page() does __wake_up_bit() which calls __wake_up() with
nr_exclusive set to 1. Doesn't that mean that only one waiter will be
woken up ?

If that's the case, then we lose because we'll have clear PG_waiters but
only wake up one of them.

Waking them all would fix it but at the risk of causing other
problems... Maybe PG_waiters need to actually be a counter but if that
is the case, then it complicates things even more.

Any smart idea ?

Ben.

-
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/