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

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Date:	Fri, 15 Jan 2016 13:11:25 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: timers: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected

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...

							Thanx, Paul