Re: cgroup trace events acquire sleeping locks

[Sebastian Andrzej Siewior]

On 2018-07-09 15:01:54 [-0400], Steven Rostedt wrote:
> > which is the trace_cgroup_rmdir() trace event in cgroup_rmdir(). The
> > trace event invokes cgroup_path() which acquires a spin_lock_t and this
> > is invoked within a preempt_disable()ed section. 
> 
> Correct. And I wish no trace event took spin locks.

is there an easy way to detect this? I mean instead hitting the trace
event with debug enabled and doing a review of each of them.

> > It says "Preemption disabled at" migrate_enable() but this is not true.
> > A printk() just before the lock reports preempt_count() of two and
> > sometimes one. I *think*
> > - one is from rcu_read_lock_sched_notrace() in __DO_TRACE()
> > - the second is from preempt_disable_notrace() in ring_buffer_lock_reserve()
> > 
> > I would prefer not to turn kernfs_rename_lock into raw_spin_lock_t. We
> > had a similar problem with a i915 trace event which eventually vanished
> > (and before I just disabled it).
> > 
> > So how likely are chances that we can use rcu_read_lock() in __DO_TRACE()?
> 
> Not very.

Is there a reason for this? I don't think this is documented. I changed
it to the "normal" RCU read section and it appeared to work :)

> > And how likely are chances that the preempt_disable() in
> > ring_buffer_lock_reserve() could be avoided while the trace event is
> > invoked?
> 
> Even less likely. The design of the ring buffer is based on not being
> able to be preempted.

I was expecting this.

> > I guess nothing of this is easy peasy. Any suggestions?
> > 
> 
> One solution, albeit not so pretty, is to move the grabbing of the
> path, outside the trace event. But this should work.

okay, wasn't aware of the trace_cgroup_##type##_enabled() magic. Yes,
this should work. Do you mind posting this upstream?

> -- Steve

Sebastian