Re: [RFC PATCH] hrtimer: remove deadlock due to waiting on IPI in softirq context

[Rik van Riel]

On 03/05/2014 04:51 PM, Thomas Gleixner wrote:
> On Wed, 5 Mar 2014, Rik van Riel wrote:
> > There appears to be a deadlock in the hrtimer code. Specifically,
> > clock_was_set() calls an IPI with wait=1, from softirq context.
>
> This should not be called from softirq context.
>
> > Waiting for IPIs to complete in irq context can lead to a deadlock,
> > because the current code (that was interrupted) might be holding some
> > kind of lock, that another CPU is waiting for with spin_lock_irq or
> > similar.
> >
> > In other words, the current CPU may need to release a resource, before
> > the IPI can be handled by one of the destination CPUs.
> >
> > To my untrained eye, it does not look like this patch introduces a
> > new bug to the timer code, but that is hard to ascertain with the
> > timer code. so I am posting this as an RFC for the timer gods to hurt
> > their brains on :)
> >
> > This bug was introduced by 54cdfdb4 in early 2007 (the original
> > hrtimer code patch).
>
> Right and we had some issues with that until we moved the calls to
> clock_was_set() out of lock held regions.

Ahh indeed, the bug got fixed already :)

> The only call which happens from interrupt context is in
> update_wall_time(). And that one definitely holds no locks which are
> relevant.
>
> On which kernel are you observing the issue?

This was RHEL6, and I saw that the immediate function
was still the same upstream.

I forgot to check that clock_was_set() is now called
in a different way. My bad.
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