linux-kernel - Re: rb tree hrtimer lockup bug (found by perf

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Date:	Thu, 24 Apr 2014 12:37:50 -0700
From:	Greg KH <greg@...ah.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Vince Weaver <vincent.weaver@...ne.edu>,
	LKML <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: rb tree hrtimer lockup bug (found by perf_fuzzer)

On Thu, Apr 17, 2014 at 09:59:01AM +0200, Thomas Gleixner wrote:
> On Wed, 16 Apr 2014, Greg KH wrote:
> > On Thu, Apr 17, 2014 at 01:00:53AM +0200, Thomas Gleixner wrote:
> > > And cdevs is an array of  struct cdev:
> > > 
> > > struct cdev {
> > > 	struct kobject kobj;
> > 
> > Those are not "real" kobjects, and are never registered with the kobject
> > core.
> > 
> > I really need to go rename those one of these days, and just make them a
> > separate object, as they have nothing to do with a "normal" kobject
> > other than the reference count and the use of the kobject map stuff.
> > 
> > So if this is showing up as a problem, something else is going on here,
> > as this should not be an issue at all.
> 
> As far as I decoded it from Vince traces the issue is only with
> DEBUG_KOBJECT_RELEASE=y. That deferres the release to a workqueue.
> 
> Now the cdevs in tty are allocated ahead and not freed on
> release. They stay in the cdevs[] array of the tty drivers.
> 
> Now if a cdev is released the kobject debug stuff schedules the
> delayed work, but the tty core can reuse the cdev.before the delayed
> work has been executed and first thing it does is calling cdev_init()
> which does a
> 
>      memset(cdev, 0, sizeof *cdev);
> 
> which of course includes the enqueued timer and the scheduled work. Go
> figure how well that goes.

Ugh, the kobject isn't "real", but it seems like it has changed over
time to be more real than I imagined.

I'll work to rip the kobject code out of the cdev soon, which should
resolve this issue, thanks.

greg k-h
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