On Thu, Jul 10, 2008 at 05:25:35PM +1000, Nick Piggin wrote: > On Wednesday 09 July 2008 07:37, Rafael J. Wysocki wrote: > > Bug-Entry : http://bugzilla.kernel.org/show_bug.cgi?id=10629 > > Subject : 2.6.26-rc1-$sha1: RIP __d_lookup+0x8c/0x160 > > Submitter : Alexey Dobriyan <adobriyan@xxxxxxxxx> > > Date : 2008-05-05 09:59 (65 days old) > > References : http://lkml.org/lkml/2008/5/5/28 > > Handled-By : Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> > > Attached is my fix for this problem. I don't think it is a regression > as such, but it can't hurt to go into 2.6.26 IMO. > > PREEMPT_RCU without HOTPLUG_CPU is broken. The rcu_online_cpu is called to > initially populate rcu_cpu_online_map with all online CPUs when the hotplug > event handler is installed, and also to populate the map with CPUs as they > come online. The former case is meant to happen with and without HOTPLUG_CPU, > but without HOTPLUG_CPU, the rcu_offline_cpu function is no-oped -- while it > still gets called, it does not set the rcu CPU map. > > With a blank RCU CPU map, grace periods get to tick by completely oblivious > to active RCU read side critical sections. This results in free-before-grace > bugs. > > Fix is obvious once the problem is known. (Also, change __devinit to > __cpuinit so the function gets thrown away on !HOTPLUG_CPU kernels). I officially feel extremely stupid. Thank you -very- much for tracking this down, Nick!!! And especially for the fix! I will give this a good testing. In the meantime: Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> > Signed-off-by: Nick Piggin <npiggin@xxxxxxx> > --- > > Annoyed this wasn't a crazy obscure error in the algorithm I could fix :) > I spent all day debugging it and had to make a special test case (rcutorture > didn't seem to trigger it), and a big RCU state logging infrastructure to log > millions of RCU state transitions and events. Oh well. > > Index: linux-2.6/kernel/rcupreempt.c > =================================================================== > --- linux-2.6.orig/kernel/rcupreempt.c 2008-07-10 17:08:56.000000000 +1000 > +++ linux-2.6/kernel/rcupreempt.c 2008-07-10 17:09:10.000000000 +1000 > @@ -925,26 +925,22 @@ void rcu_offline_cpu(int cpu) > spin_unlock_irqrestore(&rdp->lock, flags); > } > > -void __devinit rcu_online_cpu(int cpu) > -{ > - unsigned long flags; > - > - spin_lock_irqsave(&rcu_ctrlblk.fliplock, flags); > - cpu_set(cpu, rcu_cpu_online_map); > - spin_unlock_irqrestore(&rcu_ctrlblk.fliplock, flags); > -} > - > #else /* #ifdef CONFIG_HOTPLUG_CPU */ > > void rcu_offline_cpu(int cpu) > { > } > > -void __devinit rcu_online_cpu(int cpu) > +#endif /* #else #ifdef CONFIG_HOTPLUG_CPU */ > + > +void __cpuinit rcu_online_cpu(int cpu) > { > -} > + unsigned long flags; > > -#endif /* #else #ifdef CONFIG_HOTPLUG_CPU */ > + spin_lock_irqsave(&rcu_ctrlblk.fliplock, flags); > + cpu_set(cpu, rcu_cpu_online_map); > + spin_unlock_irqrestore(&rcu_ctrlblk.fliplock, flags); > +} > > static void rcu_process_callbacks(struct softirq_action *unused) > { -- To unsubscribe from this list: send the line "unsubscribe kernel-testers" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
