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SubjectRe: Query regarding synchronize_sched_expedited and resched_cpu
On Thu, Sep 21, 2017 at 03:57:49PM +0200, Peter Zijlstra wrote:
> On Mon, Sep 18, 2017 at 09:55:27AM -0700, Paul E. McKenney wrote:
> > On Mon, Sep 18, 2017 at 12:29:31PM -0400, Steven Rostedt wrote:
> > > On Mon, 18 Sep 2017 09:24:12 -0700
> > > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> > >
> > >
> > > > As soon as I work through the backlog of lockdep complaints that
> > > > appeared in the last merge window... :-(
> > > >
> > > > sparse_irq_lock, I am looking at you!!! ;-)
>
> That one is a false positive and I have send patches to address.

I did try them out, and they work fine for me when lockdep is enabled.
I get build failures if lockdep is not enabled. Things are a bit crazy
here, so I have not had a chance to try to fix them, though it should
not be a big deal (easy for me to say!).

> > > I just hit one too, and decided to write a patch to show a chain of 3
> > > when applicable.
> > >
> > > For example:
> > >
> > > Chain exists of:
> > > cpu_hotplug_lock.rw_sem --> smpboot_threads_lock --> (complete)&self->parked
> > >
> > > Possible unsafe locking scenario by crosslock:
> > >
> > > CPU0 CPU1 CPU2
> > > ---- ---- ----
> > > lock(smpboot_threads_lock);
> > > lock((complete)&self->parked);
> > > lock(cpu_hotplug_lock.rw_sem);
> > > lock(smpboot_threads_lock);
> > > lock(cpu_hotplug_lock.rw_sem);
> > > unlock((complete)&self->parked);
> > >
> > > *** DEADLOCK ***
> > >
> > > :-)
> >
> > Nice!!!
>
> That one looks like the watchdog thing, and Thomas was poking at that.

For whatever it is worth, I am still chasing lost-timer bugs. I now know
of a large number of things that are not the cause of the problem. :-/

Thanx, Paul

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