Re: perf: fuzzer BUG: KASAN: stack-out-of-bounds in __unwind_start

[Petr Mladek <pmladek@suse.com>]

On Tue 2016-11-29 09:36:00, Paul E. McKenney wrote:
> Updated (but still untested) commit below.
> 
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit d3df9bc5fb5d838b049f32a476721eadbc349553
> Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Date:   Tue Nov 29 05:49:06 2016 -0800
> 
>     rcu: Once again use NMI-based stack traces in stall warnings
>     
>     This commit is for all intents and purposes a revert of bc1dce514e9b
>     ("rcu: Don't use NMIs to dump other CPUs' stacks").  The reason to suppose
>     that this can now safely be reverted is the presence of 42a0bb3f7138
>     ("printk/nmi: generic solution for safe printk in NMI"), which is said
>     to have made NMI-based stack dumps safe.
>     
>     However, this reversion keeps one nice property of bc1dce514e9b
>     ("rcu: Don't use NMIs to dump other CPUs' stacks"), namely that
>     only those CPUs blocking the grace period are dumped.  The new
>     trigger_single_cpu_backtrace() is used to make this happen, as
>     suggested by Josh Poimboeuf.
>     
>     Reported-by: Vince Weaver <vincent.weaver@maine.edu>
>     Not-yet-signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>     Cc: Petr Mladek <pmladek@suse.com>
>     Cc: Peter Zijlstra <peterz@infradead.org>
>     Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>

Looks fine to me.

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr

> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 91a68e4e6671..ba0e4825be9d 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1396,7 +1396,10 @@ static void rcu_check_gp_kthread_starvation(struct rcu_state *rsp)
>  }
>  
>  /*
> - * Dump stacks of all tasks running on stalled CPUs.
> + * Dump stacks of all tasks running on stalled CPUs.  First try using
> + * NMIs, but fall back to manual remote stack tracing on architectures
> + * that don't support NMI-based stack dumps.  The NMI-triggered stack
> + * traces are more accurate because they are printed by the target CPU.
>   */
>  static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
>  {
> @@ -1406,11 +1409,10 @@ static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
>  
>  	rcu_for_each_leaf_node(rsp, rnp) {
>  		raw_spin_lock_irqsave_rcu_node(rnp, flags);
> -		if (rnp->qsmask != 0) {
> -			for_each_leaf_node_possible_cpu(rnp, cpu)
> -				if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu))
> +		for_each_leaf_node_possible_cpu(rnp, cpu)
> +			if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu))
> +				if (!trigger_single_cpu_backtrace(cpu))
>  					dump_cpu_task(cpu);
> -		}
>  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>  	}
>  }
>