linux-kernel - Re: [percpu] BUG: unable to handle kernel NULL pointer dereference at (null)

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Date:	Tue, 2 Sep 2014 09:21:10 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Fengguang Wu <fengguang.wu@...el.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
Subject: Re: [percpu] BUG: unable to handle kernel NULL pointer dereference
 at (null)

On Tue, Sep 02, 2014 at 11:00:12AM -0500, Christoph Lameter wrote:
> On Tue, 2 Sep 2014, Paul E. McKenney wrote:
> 
> > Before this commit, raw_cpu_add_return() didn't build.  The commit
> > didn't affect anything else.
> >
> > So I don't understand how anything could work before this commit and
> > be broken after it.  Enlightenment?
> 
> Where is that raw_cpu_add_return statement?
> 
> grep does not match anything like that in kernel/rcu/*
> and this_cpu_add_return is used extensively in the network subsystem for
> statistics.

Heh!  Good point!

Added by ac1bea85781e (sched,rcu: Make cond_resched() report RCU quiescent
states), removed by 4a81e8328d379 (rcu: Reduce overhead of cond_resched()
checks for RCU).  So, as you say, no effect on contemporary kernels.

							Thanx, Paul

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