Re: Query regarding synchronize_sched_expedited and resched_cpu

[Peter Zijlstra]

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@xxxxxxxxxxxxxxxxxx> 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 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.