Re: x86: disable preemption in delay_tsc()

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 16 Nov 2007 07:13:32 +0100 Ingo Molnar <mingo@elte.hu> wrote:

> 
> * Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> > > >     x86: disable preemption in delay_tsc()
> > > >     
> > > >     Marin Mitov points out that delay_tsc() can misbehave if it is 
> > > > preempted and rescheduled on a different CPU which has a skewed 
> > > > TSC. Fix it by disabling preemption.
> > > >  
> > > 
> > > this worries me.. this appears to effectively disable preemption 
> > > during udelay() and mdelay() loops... which are very obvious latency 
> > > inducers.
> > > 
> > > Now you can argue that if you're preemptible you should have used 
> > > msleep() and co, and I'll totally buy that.
> > > 
> > > 
> > > Maybe we should just check if we're still on the same cpu or 
> > > something, or have a cheap way to pin a process to a cpu.... but 
> > > both are longer term solutions.
> > 
> > Yes, we can do better.
> > 
> > But this bug can cause very rare failures in probably a large number 
> > of device drivers on a minorty of machines.  Ugly.  So I felt it best 
> > to plug it fast while people think about more sophisticated fixes.
> 
> how about using usleep() transparently if high-res timers are active and 
> we have !preempt_count()?

And CONFIG_PREEMPT, of course

> That would be a sufficient solution and would 
> avoid all the calibration and per-cpu-ness problems.

It sounds like it would work OK.  What is the setup cost for a usleep?  I'd
have thought that code which does something like

	while (i++ < 1000) {
		foo();
		udelay(1);
	}

would take qiute a bit longer with such a change?