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    SubjectRe: [PATCH] ia64: disable preemption in udelay()
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    On Thu, 2005-12-15 at 17:52 -0800, Zwane Mwaikambo wrote:
    > 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.

    There are 10 drivers that udelay(10000) or more and a TON that
    udelay(1000). Turning those all into 1ms+ non preemptible sections will
    be very bad.

    Lee

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