linux-kernel - Re: cpufreq/ondemand: unpinning an unpinned lock.

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Message-ID: <20150722090256.GP25159@twins.programming.kicks-ass.net>
Date:	Wed, 22 Jul 2015 11:02:56 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Dave Jones <davej@...emonkey.org.uk>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-pm@...r.kernel.org
Subject: Re: cpufreq/ondemand: unpinning an unpinned lock.

On Tue, Jul 21, 2015 at 05:46:16PM -0700, Stephen Boyd wrote:
> On 07/16/2015 05:42 AM, Dave Jones wrote:
> >On Thu, Jul 16, 2015 at 10:41:50AM +0530, Viresh Kumar wrote:
> >
> >  > > > WARNING: CPU: 1 PID: 29529 at kernel/locking/lockdep.c:3497 lock_unpin_lock+0x109/0x110()
> >  > > > unpinning an unpinned lock
> >  > > > Call Trace:

> >  > > >  [<ffffffffae0d0ec9>] lock_unpin_lock+0x109/0x110
> >  > > >  [<ffffffffae7f93cc>] __schedule+0x3ac/0xb60
> >  > > >  [<ffffffffae7f9c41>] schedule+0x41/0x90
> >  > > >  [<ffffffffae7f9ff8>] schedule_preempt_disabled+0x18/0x30

> Lock pinning is new in v4.2-rc1. Adding Peter just in case there's some
> insight.

Hmm, weird. I've not seen it happen before.

There's potentially 3 unpin's in __schedule():

  - the obvious one in the prev == next case,
  - the on in context_switch(),
  - the on in try_to_wake_up_local().

All 3 appear to be balanced; the first and second against the pin_lock()
at the start of __schedule() and the third against that and it repins
the lock against the first two.

Most curious.
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