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

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Date:	Tue, 3 Jun 2008 01:03:17 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Nick Piggin <npiggin@...e.de>, David Howells <dhowells@...hat.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/3] 64-bit futexes: Intro


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

> > 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).
> 
> No, "unlocks as a natural barrier" only works for exclusive kernel 
> locks (spin_unlock and write_unlock). There we can just do a write to 
> unlock. But for anything that wants to handle contention differently 
> than just spinning, the unlock path needs to be able to do an atomic 
> "unlock and test if I need to do something else", because it may need 
> to wake things up.

yeah, indeed. Compared to all the other costs that have to be dealt with 
here, having a second atomic op isnt all that much of an issue either, 
especially on latest hw. An atomic op will probably never be as cheap as 
a non-atomic op, but ~20 cycles is still plenty fast for most practical 
purposes.

	Ingo
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