linux-kernel - Re: [rcu] BUG: unable to handle kernel NULL pointer dereference at 000000da

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Message-ID: <20140913003837.GO4775@linux.vnet.ibm.com>
Date:	Fri, 12 Sep 2014 17:38:37 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Fengguang Wu <fengguang.wu@...el.com>
Cc:	Christoph Lameter <cl@...ux.com>, Shan Wei <davidshan@...cent.com>,
	Jet Chen <jet.chen@...el.com>, Su Tao <tao.su@...el.com>,
	Yuanhan Liu <yuanhan.liu@...el.com>, LKP <lkp@...org>,
	linux-kernel@...r.kernel.org, bobby.prani@...il.com,
	Tejun Heo <htejun@...il.com>
Subject: Re: [rcu] BUG: unable to handle kernel NULL pointer dereference at
 000000da

On Sat, Sep 13, 2014 at 08:20:05AM +0800, Fengguang Wu wrote:
> On Fri, Sep 12, 2014 at 12:26:59PM -0700, Paul E. McKenney wrote:
> > On Fri, Sep 12, 2014 at 02:19:57PM -0500, Christoph Lameter wrote:
> > > On Fri, 12 Sep 2014, Paul E. McKenney wrote:
> > > 
> > > > So, I am not seeing this failure in my testing, but my best guess is
> > > > that the problem is due to the fact that force_quiescent_state() is
> > > > sometimes invoked with preemption enabled, which breaks __this_cpu_read()
> > > > though perhaps with very low probability.  The common-case call (from
> > > > __call_rcu_core()) -does- have preemption disabled, in fact, it has
> > > > interrupts disabled.
> > > 
> > > How could __this_cpu_read() break in a way that would make a difference to
> > > the code? There was no disabling/enabling of preemption before the patch
> > > and there is nothing like that after the patch. If there was a race then
> > > it still exists. The modification certainly cannot create a race.
> > 
> > Excellent question.  Yet Fengguang's tests show breakage.
> > 
> > Fengguang, any possibility of a false positive here?
> 
> Yes, it is possible. I find the first bad commit and its parent
> commit's kernels are built in 2 different machines which might
> cause subtle changes. I'll redo the bisect.

Thank you, Fengguang, and please let me know how it goes!

							Thanx, Paul

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