Re: [PATCH] cpu/hotplug: Reset node state after operation

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sat, Oct 21, 2017 at 04:06:52PM +0200, Thomas Gleixner wrote:
> The recent rework of the cpu hotplug internals changed the usage of the per
> cpu state->node field, but missed to clean it up after usage.
> 
> So subsequent hotplug operations use the stale pointer from a previous
> operation and hand it into the callback functions. The callbacks then
> dereference a pointer which either belongs to a different facility or
> points to freed and potentially reused memory. In either case data
> corruption and crashes are the obvious consequence.
> 
> Reset the node and the last pointers in the per cpu state to NULL after the
> operation which set them has completed.
> 
> Fixes: 96abb968549c ("smp/hotplug: Allow external multi-instance rollback")
> Reported-by: Tvrtko Ursulin <tursulin@ursulin.net>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

This does fine with rcutorture, though it does not fix the lost
(or egregiously delayed) timer problem that is still eluding me.
Nevertheless, for whatever it is worth:

Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> ---
>  kernel/cpu.c |    5 +++++
>  1 file changed, 5 insertions(+)
> 
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -632,6 +632,11 @@ cpuhp_invoke_ap_callback(int cpu, enum c
>  		__cpuhp_kick_ap(st);
>  	}
> 
> +	/*
> +	 * Clean up the leftovers so the next hotplug operation wont use stale
> +	 * data.
> +	 */
> +	st->node = st->last = NULL;
>  	return ret;
>  }
> 
>