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

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Message-ID: <alpine.DEB.2.11.1409121413540.21514@gentwo.org>
Date:	Fri, 12 Sep 2014 14:19:57 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Fengguang Wu <fengguang.wu@...el.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 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.


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