linux-kernel - Re: [PATCH 0/3] 64-bit futexes: Intro

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: <20080602185433.GB4081@elte.hu>
Date:	Mon, 2 Jun 2008 20:54:33 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ulrich Drepper <drepper@...hat.com>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, mtk.manpages@...il.com
Subject: Re: [PATCH 0/3] 64-bit futexes: Intro

* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> IOW, I'm faking it, but I'm making a point. Namely that you can 
> efficiently do read-write lock using *only* 32-bit ops, and without 
> any real kind of limitations on the number of readers and writers.
> 
> So here goes the explanation and the pseudo-code.
> 
>  - have two levels of locking: the contended case, and the uncontended
>    case

i suspect _any_ abstract locking functionality around a data structure 
can be implemented via atomic control over just a single user-space bit.

That bit can be used as a lock and if all access to the state of that 
atomic variable uses it, arbitrary higher-order atomic state transitions 
can be derived from it. The cost would be a bit more instructions in the 
fastpath, but there would still only be a single atomic op (the acquire 
op), as the unlock would be a natural barrier (on x86 at least).

Concurrency (and scheduling) of that lock would still be exactly the 
same as with genuine 64-bit (or even larger) atomic ops, and the 
fastpath would be very close as well.

	Ingo
--
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/