Re: x86: disable preemption in delay_tsc()

[Ingo Molnar]

* Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> 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()? That would be a sufficient solution and would 
avoid all the calibration and per-cpu-ness problems.

	Ingo
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