Re: [PATCH 1/2] tracing: Have stack tracer force RCU to be watching

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

On Thu, Oct 22, 2015 at 09:20:45AM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>
> 
> The stack tracer was triggering the WARN_ON() in module.c:
> 
>  static void module_assert_mutex_or_preempt(void)
>  {
>  #ifdef CONFIG_LOCKDEP
> 	if (unlikely(!debug_locks))
> 		return;
> 
> 	WARN_ON(!rcu_read_lock_sched_held() &&
> 		!lockdep_is_held(&module_mutex));
>  #endif
>  }
> 
> The reason is that the stack tracer traces all function calls, and some of
> those calls happen while exiting or entering user space and idle. Some of
> these functions are called after RCU had already stopped watching, as RCU
> does not watch userspace or idle CPUs.
> 
> If a max stack is hit, then the save_stack_trace() is called, which will
> check module addresses and call module_assert_mutex_or_preempt(), and then
> trigger the warning. Sad part is, the warning itself will also do a stack
> trace and tigger the same warning. That probably should be fixed.
> 
> The warning was added by 0be964be0d45 "module: Sanitize RCU usage and
> locking" but this bug has probably been around longer. But it's unlikely to
> cause much harm, but the new warning causes the system to lock up.
> 
> Cc: stable@vger.kernel.org # 4.2+
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc:"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

It does look like it will make RCU pay attention in the region of interest.

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

> ---
>  kernel/trace/trace_stack.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
> index b746399ab59c..5f29402bff0f 100644
> --- a/kernel/trace/trace_stack.c
> +++ b/kernel/trace/trace_stack.c
> @@ -88,6 +88,12 @@ check_stack(unsigned long ip, unsigned long *stack)
>  	local_irq_save(flags);
>  	arch_spin_lock(&max_stack_lock);
> 
> +	/*
> +	 * RCU may not be watching, make it see us.
> +	 * The stack trace code uses rcu_sched.
> +	 */
> +	rcu_irq_enter();
> +
>  	/* In case another CPU set the tracer_frame on us */
>  	if (unlikely(!frame_size))
>  		this_size -= tracer_frame;
> @@ -169,6 +175,7 @@ check_stack(unsigned long ip, unsigned long *stack)
>  	}
> 
>   out:
> +	rcu_irq_exit();
>  	arch_spin_unlock(&max_stack_lock);
>  	local_irq_restore(flags);
>  }
> -- 
> 2.6.1
> 
>