linux-kernel - Re: rwsem: down_read

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:	Wed, 5 May 2010 03:36:46 -0700
From:	Michel Lespinasse <walken@...gle.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>, Ying Han <yinghan@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: rwsem: down_read_unfair() proposal

On Wed, May 05, 2010 at 11:03:40AM +0100, David Howells wrote:
> If the system is as heavily loaded as you say, how do you prevent
> writer starvation?  Or do things just grind along until sufficient
> threads are queued waiting for a write lock?

Reader/Writer fairness is not disabled in the general case - it only is
for a few specific readers such as /proc/<pid>/maps. In particular, the
do_page_fault path, which holds a read lock on mmap_sem for potentially long
(~disk latency) periods of times, still uses a fair down_read() call.
In comparison, the /proc/<pid>/maps path which we made unfair does not
normally hold the mmap_sem for very long (it does not end up hitting disk);
so it's been working out well for us in practice.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
--
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/