linux-kernel - Re: [patch] change futex

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Message-ID: <20070312122103.GA30778@elte.hu>
Date:	Mon, 12 Mar 2007 13:21:03 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Roland McGrath <roland@...hat.com>, akpm@...ux-foundation.org,
	mm-commits@...r.kernel.org, drepper@...hat.com, oleg@...sign.ru,
	sebastien.dugue@...l.net, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch] change futex_wait() to hrtimers


* Nick Piggin <npiggin@...e.de> wrote:

> > > > the issue is this: your fix reduces the effects of the bug but 
> > > > it is still fundamentally incomplete because of the use of 
> > > > timer_list. So
> > > 
> > > But using schedule_timeout is not a bug. Userspace timeouts are 
> > > always defined to be "at least".
> > 
> > but what you are adding isnt a plain schedule_timeout(), it is a 
> > restart block handling loop. And for those restart blocks that 
> > relate to timeouts, we only use hrtimers. I am not making this up to 
> > annoy you: take a look at all the current restart block handlers - 
> > they are hrtimer based, for exactly this reason.
> 
> So why do you say it is fundamentally incomplete?

because i misread your last patch :-) I thought it still has a window 
for inaccuracy, but you are right: it should be at most 1 jiffy 
inaccurate, no matter how many times we restart.

still ... the hrtimers patch has been submitted to lkml before yours, 
and has been tested extensively, so why go the extra side-jump 
prolonging the jiffies sleep method? The LTP failure has been there 
since the inception of the futex code i suspect. Going this way also 
enables the addressing of a more pressing need: the elimination of 
glibc's forced use of relative futex timeouts.

	Ingo
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