Re: timers: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Jan 15, 2016 at 01:11:25PM -0800, Paul E. McKenney wrote:
> On Fri, Jan 15, 2016 at 11:03:24AM +0100, Thomas Gleixner wrote:
> > On Thu, 14 Jan 2016, Paul E. McKenney wrote:
> > > > Untested patch below.
> > > 
> > > One small fix to make it build below.  Started rcutorture, somewhat
> > > pointlessly given that the splat doesn't appear on my setup.
> > 
> > Well, at least it tells us whether the change explodes by itself.
> 
> Hmmm...
> 
> So this is a strange one.  I have been seeing increasing instability
> in mainline over the past couple of releases, with the main symptom
> being that the kernel decides that awakening RCU's grace-period kthreads
> is an optional activity.  The usual situation is that the kthread is
> blocked for tens of seconds in an wait_event_interruptible_timeout(),
> despite having a three-jiffy timeout.  Doing periodic wakeups from
> the scheduling-clock interrupt seems to clear things up, but such hacks
> should not be necessary.
> 
> Normally, I have to run for for some hours to have a good chance of seeing
> this happen.  This change triggered in a 30-minute run.  Not only that,
> but in a .config scenario that is normally very hard to trigger.  This
> scenario does involve CPU hotplug, and I am re-running with CPU hotplug
> disabled.
> 
> That said, I am starting to hear reports of people hitting this without
> CPU hotplug operations...

And without hotplug operations, instead of dying repeatedly in 30 minutes,
it goes four hours with no complaints.  Next trying wakeups.

							Thanx, Paul