Re: [RFC 0/3] Revert SRCU from tracepoint infrastructure

[Joel Fernandes <joel@joelfernandes.org>]

Hi Steve,

On Mon, Feb 10, 2020 at 03:03:48PM -0500, Steven Rostedt wrote:
> On Mon, 10 Feb 2020 14:53:02 -0500
> Joel Fernandes <joel@joelfernandes.org> wrote:
> 
> > > diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> > > index 1fb11daa5c53..a83fd076a312 100644
> > > --- a/include/linux/tracepoint.h
> > > +++ b/include/linux/tracepoint.h
> > > @@ -179,10 +179,8 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
> > >  		 * For rcuidle callers, use srcu since sched-rcu	\
> > >  		 * doesn't work from the idle path.			\
> > >  		 */							\
> > > -		if (rcuidle) {						\
> > > +		if (rcuidle)						\
> > >  			__idx = srcu_read_lock_notrace(&tracepoint_srcu);\
> > > -			rcu_irq_enter_irqson();				\
> > > -		}							\  
> > 
> > This would still break out-of-tree modules or future code that does
> > rcu_read_lock() right in a tracepoint callback right?
> 
> Yes, and that's fine.
> 
> > 
> > Or are we saying that rcu_read_lock() in a tracepoint callback is not
> > allowed? I believe this should then at least be documented somewhere.  Also,
> 
> No, it's only not allowed if you you attached to a tracepoint that can
> be called without rcu watching. That's up to the caller to figure it
> out. Tracepoints were never meant to be a generic thing people should
> use without knowing what they are really doing.

Ok, right.

> > what about code in tracepoint callback that calls rcu_read_lock() indirectly
> > through a path in the kernel, and also code that may expect RCU readers when
> > doing preempt_disable()?
> 
> Then they need to know what they are doing.

Ok.

> > So basically we are saying with this patch:
> > 1. Don't call in a callback: rcu_read_lock() or preempt_disable() and expect RCU to do
> > anything for you.
> 
> We can just say, "If you plan on using RCU, be aware that it man not be
> watching and you get do deal with the fallout. Use rcu_is_watching() to
> figure it out."

Ok.

> > 2. Don't call code that does anything that 1. needs.
> > 
> > Is that intended? thanks,
> > 
> 
> No, look what the patch did for perf. Why make *all* callbacks suffer
> if only some use RCU? If you use RCU from a callback, then you need to
> figure it out. The same goes for attaching to the function tracer.

Only the callbacks on the rcuidle ones would suffer though, not all
callbacks.

Yes I saw the patch, it looks like a good idea to me and I am Ok with it.

thanks,

 - Joel