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

[Paul E. McKenney]

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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/