Re: timer: lockup in run_timer_softirq()

[Steven Rostedt]

On Wed, 2013-07-10 at 11:52 +0200, Peter Zijlstra wrote:

> Fun.. :-) we trace __local_bh_enable() and hit a ftrace callback between
> telling lockdep we enabled softirqs and actually doing so.
> 
> I'm just a tad confused by the trace; it says we go:
>   lock_is_held()
>     check_flags()
> 
> Looking at perf_tp_event() this would most likely be from:
> 
>   ctx = rcu_dereference(task->perf_event_ctxp[perf_sw_context]);

Function tracing should not be treated as a normal trace point. It is
much more invasive, and there's things that one should be weary about
when using it.

> 
> Where the lock_is_held() would be from rcu_dereference_check()'s
> rcu_read_lock_sched_held(). However, by there we've already passed
> rcu_read_lock() which includes rcu_lock_acquire() -> lock_acquire() ->
> check_flags(). So it should've triggered there.
> 
> Ideally we'd not trace __local_bh_enable() at all, seeing as how any RCU usage
> in there would be bound to trigger this.

I find it very useful to trace __local_bh_enable(). I also trace RCU
calls.

When using function tracing, you need to use
rcu_dereference_raw_notrace().

Also, function tracing callbacks should avoid rcu_read_lock(), as that's
traced as well. You can use preempt_disable_notrace() for rcu usage.

-- Steve


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