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Message-ID: <20140913002005.GA9550@localhost> Date: Sat, 13 Sep 2014 08:20:05 +0800 From: Fengguang Wu <fengguang.wu@...el.com> To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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