From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Christoph Lameter <cl@linux.com>
Cc: Fengguang Wu <fengguang.wu@intel.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: Fri, 12 Sep 2014 12:26:59 -0700 [thread overview]
Message-ID: <20140912192659.GM4775@linux.vnet.ibm.com> (raw)
In-Reply-To: <alpine.DEB.2.11.1409121413540.21514@gentwo.org>
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?
Thanx, Paul
next prev parent reply other threads:[~2014-09-12 19:27 UTC|newest]
Thread overview: 8+ 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
2014-09-12 19:02 ` Paul E. McKenney
2014-09-12 19:19 ` Christoph Lameter
2014-09-12 19:26 ` Paul E. McKenney [this message]
2014-09-13 0:20 ` Fengguang Wu
2014-09-13 0:38 ` Paul E. McKenney
2014-09-13 12:38 ` Fengguang Wu
2014-09-12 20:39 ` Pranith Kumar
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