linux-kernel - Re: [patch] change futex

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Message-ID: <20070312111331.GF28546@wotan.suse.de>
Date:	Mon, 12 Mar 2007 12:13:31 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Ingo Molnar <mingo@...e.hu>
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

On Mon, Mar 12, 2007 at 12:02:04PM +0100, Ingo Molnar wrote:
> 
> * Nick Piggin <npiggin@...e.de> wrote:
> 
> > > i dont think we should try to do this. We should not and cannot do 
> > > anything about all of the artifacts that comes with the use of 
> > > relative timeouts and schedule_timeout().
> > > 
> > > basically, using jiffies here (which schedule_timeout() does) is 
> > > /fundamentally/ imprecise. If you get many interrupts, rounding 
> > > errors sum up - and there's nothing we can do about it!
> > 
> > Well I did convert futex_wait to an absolute timeout based version in 
> > the subsequent incremental patch. I think that is OK?
> 
> it still has the rounding artifacts: using timer_list there is no way to 
> do a precise long sleep based on many small sleeps.

OK but that is nothing to do with my patch, but the original futex_wait
implementation.

> even if this means more work for you (i'm sorry about that!) i'm quite 
> sure we should take Sebastien's hrtimers based implementation of 
> futex_wait(), and use the nanosleep method to restart it. There's no 
> point in further tweaking the imprecise approach: whenever some timeout 
> needs to be restarted, it's a candidate for hrtimers.

Absolute timeout is needed, sure. But once that is done, hrtimers does
not fix a bug, does it?

> 
> until then, glibc already handles timeouts and restarts it manually.

It isn't timeout handling that is buggy, but EINTR behaviour. And
glibc does not handle that here.

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