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

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?