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From: Fengguang Wu <fengguang.wu@intel.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Christoph Lameter <cl@linux.com>,
	Shan Wei <davidshan@tencent.com>, Jet Chen <jet.chen@intel.com>,
	Su Tao <tao.su@intel.com>, Yuanhan Liu <yuanhan.liu@intel.com>,
	LKP <lkp@01.org>,
	linux-kernel@vger.kernel.org, bobby.prani@gmail.com,
	Tejun Heo <htejun@gmail.com>
Subject: Re: [rcu] BUG: unable to handle kernel NULL pointer dereference at 000000da
Date: Sat, 13 Sep 2014 08:20:05 +0800	[thread overview]
Message-ID: <20140913002005.GA9550@localhost> (raw)
In-Reply-To: <20140912192659.GM4775@linux.vnet.ibm.com>

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.

Thanks,
Fengguang

WARNING: multiple messages have this Message-ID (diff)
From: Fengguang Wu <fengguang.wu@intel.com>
To: lkp@lists.01.org
Subject: Re: [rcu] BUG: unable to handle kernel NULL pointer dereference at 000000da
Date: Sat, 13 Sep 2014 00:20:43 +0000	[thread overview]
Message-ID: <20140913002005.GA9550@localhost> (raw)
In-Reply-To: <20140912192659.GM4775@linux.vnet.ibm.com>

[-- Attachment #1: Type: text/plain, Size: 1241 bytes --]

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.

Thanks,
Fengguang

  reply	other threads:[~2014-09-13  0:20 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-09-01  8:44 [rcu] BUG: unable to handle kernel NULL pointer dereference at 000000da Fengguang Wu
2014-09-01  8:44 ` Fengguang Wu
2014-09-12 19:02 ` Paul E. McKenney
2014-09-12 19:02   ` Paul E. McKenney
2014-09-12 19:19   ` Christoph Lameter
2014-09-12 19:20     ` Christoph Lameter
2014-09-12 19:26     ` Paul E. McKenney
2014-09-12 19:27       ` Paul E. McKenney
2014-09-13  0:20       ` Fengguang Wu [this message]
2014-09-13  0:20         ` Fengguang Wu
2014-09-13  0:38         ` Paul E. McKenney
2014-09-13  0:38           ` Paul E. McKenney
2014-09-13 12:38           ` Fengguang Wu
2014-09-13 12:39             ` Fengguang Wu
2014-09-12 20:39   ` Pranith Kumar
2014-09-12 20:41     ` Pranith Kumar

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