linux-kernel - Re: unpinning an unpinned lock. (pidns/scheduler)

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Date:	Sun, 2 Aug 2015 18:03:01 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Dave Jones <davej@...emonkey.org.uk>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>
Subject: Re: unpinning an unpinned lock. (pidns/scheduler)

On Sun, Aug 02, 2015 at 11:38:07AM -0400, Dave Jones wrote:
> On Fri, Jul 31, 2015 at 01:43:53PM -0400, Dave Jones wrote:
>  > Just found a machine with this on 4.2-rc4
>  > 
>  > WARNING: CPU: 0 PID: 11787 at kernel/locking/lockdep.c:3497 lock_unpin_lock+0x109/0x110()
>  > unpinning an unpinned lock
>  > CPU: 0 PID: 11787 Comm: kworker/0:1 Not tainted 4.2.0-rc4-think+ #5
>  > Workqueue: events proc_cleanup_work
>  >  0000000000000009 ffff8804f8983988 ffffffff9f7f5eed 0000000000000007
>  >  ffff8804f89839d8 ffff8804f89839c8 ffffffff9f07b72a 00000000000000a8
>  >  0000000000000070 ffff8805079d5c98 0000000000000092 0000000000000002
>  > Call Trace:
>  >  [<ffffffff9f7f5eed>] dump_stack+0x4f/0x7b
>  >  [<ffffffff9f07b72a>] warn_slowpath_common+0x8a/0xc0
>  >  [<ffffffff9f07b7a6>] warn_slowpath_fmt+0x46/0x50
>  >  [<ffffffff9f0d0c59>] lock_unpin_lock+0x109/0x110
>  >  [<ffffffff9f7f944f>] __schedule+0x39f/0xb30
>  >  [<ffffffff9f7f9ca1>] schedule+0x41/0x90

>  > ---[ end trace e75342db87128aeb ]---
> 
> I'm hitting this a few times a day now, I'll see if I can narrow down
> a reproducer next week.   Adding the RCU cabal to Cc.

Thanks, I got a report from Sasha as well, but so far I'm a bit
stumped on how this can happen.

Clearly there's an imbalance in some unlikely scheduler path, but so far
its eluding me :/
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