Re: [PATCH] ia64: disable preemption in udelay()

[Zwane Mwaikambo <zwane@arm.linux.org.uk>]

On Thu, 15 Dec 2005, Luck, Tony wrote:

> On Wed, Dec 14, 2005 at 03:25:26PM -0800, hawkes@sgi.com wrote:
> > Sending this to a wider audience:
> > 
> > The udelay() inline for ia64 uses the ITC.  If CONFIG_PREEMPT is enabled
> > and the platform has unsynchronized ITCs and the calling task migrates
> > to another CPU while doing the udelay loop, then the effective delay may
> > be too short or very, very long.
> > 
> > The most simple fix is to disable preemption around the udelay looping.
> > The downside is that this inhibits realtime preemption for cases of long
> > udelays.  One datapoint:  an SGI realtime engineer reports that if
> > CONFIG_PREEMPT is turned off, that no significant holdoffs are
> > are attributed to udelay().
> > 
> > I am reluctant to propose a much more complicated patch (that disables
> > preemption only for "short" delays, and uses the global RTC as the time
> > base for longer, preemptible delays) unless this patch introduces
> > significant and unacceptable preemption delays.
> 
> Stuck between a rock and the proverbial hard place.
> 
> I think that the more complex patch is needed though.  If some crazy
> driver has a pre-emptible udelay(10000), then you really don't want
> to spin for that long without allowing preemption.

If it's a preemptible sleep period it should just use msleep.
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