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]

Date:	Fri, 30 May 2008 21:23:31 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ulrich Drepper <drepper@...hat.com>
cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	mtk.manpages@...il.com
Subject: Re: [PATCH 0/3] 64-bit futexes: Intro



On Fri, 30 May 2008, Linus Torvalds wrote:
> 
> Also note how you can use 64-bit atomic ops to do that all in user space, 
> without actually requiring 64-bit futex support - as long as the bits that 
> matter for waking (like "was there more than one pending writer") fit in 
> the one 32-bit word.

So here's an example of this:

 - make all the readers/writers actually update a 64-bit word (by using 
   cmpxchg8b in user space to actually get the locks)

 - but organize things so that a reader only needs to look at the high 32 
   bits to actually make it's wakeup-decision, and a sleeping writer only 
   needs to look at the low 32 bits. How? Make the low bits of the words 
   contain the "contention status".

Is it possible? I dunno. I personally suspect it is. I also suspect you 
didn't even try.

			Linus
--
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/