On Thu, Oct 22, 2015 at 09:20:45AM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxx # 4.2+
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc:"Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
It does look like it will make RCU pay attention in the region of interest.
Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
> 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
>
>
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