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    SubjectRe: 2.6.26-rc9-git4: Reported regressions from 2.6.25
    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@gmail.com>
    > > Date : 2008-05-05 09:59 (65 days old)
    > > References : http://lkml.org/lkml/2008/5/5/28
    > > Handled-By : Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    >
    > 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@linux.vnet.ibm.com>

    > Signed-off-by: Nick Piggin <npiggin@suse.de>
    > ---
    >
    > 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)
    > {



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