Re: [PATCH tip/core/rcu 04/13] rcu: Don't disable preemption for Tiny and Tree RCU readers

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

On Tue, Oct 06, 2015 at 10:05:38PM +0200, Peter Zijlstra wrote:
> On Tue, Oct 06, 2015 at 10:42:04AM -0700, Paul E. McKenney wrote:
> 
> > 
> > Ah.  The reason is that Tiny RCU and Tree RCU (the !PREEMPT ones) act
> > by implicitly extending (and, if need be, merging) the RCU read-side
> > critical sections to include all the code between successive quiescent
> > states, for example, all the code between a pair of calls to schedule().
> > 
> > Therefore, there need to be barrier() calls in the quiescent-state
> > functions.  Some could be argued to be implicitly present due to
> > translation-unit boundaries, but paranoia and all that.
> > 
> > Would adding that sort of explanation help?
> 
> > +++ b/include/linux/rcutiny.h
> > @@ -216,6 +216,7 @@ static inline bool rcu_is_watching(void)
> >  
> >  static inline void rcu_all_qs(void)
> >  {
> > +	barrier(); /* Avoid RCU read-side critical sections leaking across. */
> >  }
> >  
> >  #endif /* __LINUX_RCUTINY_H */
> 
> This is more than sheer paranoia I think, inlined functions are not a
> compiler barrier.

Yep, agreed.

> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index b9d9e0249e2f..93c0f23c3e45 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -337,12 +337,14 @@ static void rcu_momentary_dyntick_idle(void)
> >   */
> >  void rcu_note_context_switch(void)
> >  {
> > +	barrier(); /* Avoid RCU read-side critical sections leaking down. */
> >  	trace_rcu_utilization(TPS("Start context switch"));
> >  	rcu_sched_qs();
> >  	rcu_preempt_note_context_switch();
> >  	if (unlikely(raw_cpu_read(rcu_sched_qs_mask)))
> >  		rcu_momentary_dyntick_idle();
> >  	trace_rcu_utilization(TPS("End context switch"));
> > +	barrier(); /* Avoid RCU read-side critical sections leaking up. */
> >  }
> >  EXPORT_SYMBOL_GPL(rcu_note_context_switch);
> 
> These OTOH could be fixed with a noinline, such that the compiler may
> never inline it, even with whole-program-optimizations, thereby
> guaranteeing a function call boundary or compiler barrier.

I like the barrier() with the comment.  I expect it will be a bit more
robust against toolchain changes.

							Thanx, Paul