Date:	Sat, 7 Sep 2013 18:55:10 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, sbw@....edu
Subject: Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical
 section?

On Fri, Sep 06, 2013 at 09:19:30PM -0400, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@...ux.vnet.ibm.com) wrote:
> > On Fri, Sep 06, 2013 at 02:21:35PM -0400, Steven Rostedt wrote:
> > > On Fri, 6 Sep 2013 10:52:38 -0700
> > > "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> > > 
> > > > > What exactly does "extended quiescent state" mean? (Note, that's a
> > > > > rhetorical question)
> > > > 
> > > > In which case my rhetorical (and therefore useless) answer has to be
> > > > "it is a quiescent state that is extended".  ;-)
> > > > 
> > > > Sorry, couldn't resist...
> > > 
> > > Of course you couldn't ;)
> > > 
> > > > 
> > > > > I wonder if we should change "rcu_cpu_ignore()" for "rcu_eqs_enter()"
> > > > > and "rcu_cpu_heed()" for "rcu_eqs_exit()", as IMHO that's much more
> > > > > straight forward to understand than trying to wrap you head around what
> > > > > a quiescent state is, and why we are entering it or exiting it.
> > > > > 
> > > > > It also flat out explains to people that rcu is not processing that
> > > > > current CPU, and things like rcu_read_lock() should not be used.
> > > > > 
> > > > > Then we can say "rcu_cpu_is_ignored()" for things like
> > > > > "rcu_is_cpu_eqs()".
> > > > 
> > > > Currently, none of RCU's _eqs functions are exported, so they have
> > > > the potential to confuse only people working on the RCU implementation
> > > > itself, who had better understand what "eqs" means.
> > > 
> > > Yeah, that's what I thought, and never cared about the "eqs" meaning.
> > > 
> > > > 
> > > > But I do count your vote against "eqs" appearing in the name of any
> > > > function exported by RCU.
> > > 
> > > Right, their shouldn't be any "eqs" functions that are global to users
> > > outside of the RCU infrastructure.
> > > 
> > > > 
> > > > How about if I made rcu_is_cpu_idle() be as follows?
> > > > 
> > > > int rcu_is_cpu_idle(void)
> > > > {
> > > > 	int ret;
> > > > 
> > > > 	ret = (atomic_read(&per_cpu(rcu_dynticks.dynticks,
> > > > 				    raw_smp_processor_id())) & 0x1) == 0;
> > > > 	return ret;
> > > > }
> > > > 
> > > > This should allow existing uses to function properly and should allow
> > > > you to use it as well.
> > > >
> > > 
> > > You already said it wont work, but I still would have been against
> > > using it, because I wouldn't be checking if rcu thinks the CPU is idle,
> > > as NO_HZ_FULL has nothing to do with idle.
> > 
> > OK then, how about the following?
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > rcu: Is it safe to enter an RCU read-side critical section?
> > 
> > There is currently no way for kernel code to determine whether it
> > is safe to enter an RCU read-side critical section, in other words,
> > whether or not RCU is paying attention to the currently running CPU.
> > Given the large and increasing quantity of code shared by the idle loop
> > and non-idle code, the this shortcoming is becoming increasingly painful.
> > 
> > This commit therefore adds rcu_watching_this_cpu(), which returns true
> > if it is safe to enter an RCU read-side critical section on the currently
> > running CPU.  This function is quite fast, using only a __this_cpu_read().
> > However, the caller must disable preemption.
> 
> Hi Paul,
> 
> Hopefully I won't be redundant with other prior comments, but how about
> the following:

The naming discussion has been a bit fraught, so I was focusing initially
on the functionality.  Which still isn't quite right.

> static inline __rcu_read_check(void):
>   - checks if it is safe to enter a RCU read-side critical section in
>     the current context.
>   - requires that the caller disable preemption.
> 
> static inline rcu_read_check(void):
>   - disables preemption and inlines __rcu_read_check().

My fear, rightly or wrongly, is that this would get conflated with
rcu_dereference_check().

> I don't think it is semantically a good thing to bury the
> implementation-specific detail (whether is RCU watched on this
> particular CPU) into the API naming. Also, I think the generic version
> of this check should require no "special knowledge" from the user, hence
> my double-underscores proposal for the optimized version.

I do agree that the "__" vs. no-"__" distinction does make a lot of sense.
Especially now that I have them checking the same condition.  Which they
need to be, or Steve might get false positives when tracing some of
RCU's functions on the path to/from idle.

That said, I would say that "RCU watching this CPU" is not so much
implementation-specific as it is the semantic meaning.  I could imagine
other variations on this theme, such as "CPU under RCU control",
"CPU safe for RCU readers", and so on, but "RCU watching this CPU"
seems a bit more direct.

But please tell me more about what you have in mind here.

							Thanx, Paul

> Thoughts ?
> 
> Thanks,
> 
> Mathieu
> 
> > 
> > Reported-by: Steven Rostedt <rostedt@...dmis.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 5b444e0..a41eb35 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -261,6 +261,10 @@ static inline void rcu_user_hooks_switch(struct task_struct *prev,
> >  		rcu_irq_exit(); \
> >  	} while (0)
> >  
> > +#if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) || defined(CONFIG_SMP)
> > +extern int rcu_is_cpu_idle(void);
> > +#endif /* #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) || defined(CONFIG_SMP) */
> > +
> >  /*
> >   * Infrastructure to implement the synchronize_() primitives in
> >   * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
> > @@ -297,10 +301,6 @@ static inline void destroy_rcu_head_on_stack(struct rcu_head *head)
> >  }
> >  #endif	/* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
> >  
> > -#if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_SMP)
> > -extern int rcu_is_cpu_idle(void);
> > -#endif /* #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_SMP) */
> > -
> >  #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU)
> >  bool rcu_lockdep_current_cpu_online(void);
> >  #else /* #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
> > diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
> > index e31005e..67fe672 100644
> > --- a/include/linux/rcutiny.h
> > +++ b/include/linux/rcutiny.h
> > @@ -132,4 +132,13 @@ static inline void rcu_scheduler_starting(void)
> >  }
> >  #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
> >  
> > +#ifdef CONFIG_RCU_TRACE
> > +
> > +static inline bool rcu_watching_this_cpu(void)
> > +{
> > +	return !rcu_is_cpu_idle();
> > +}
> > +
> > +#endif /* #ifdef CONFIG_RCU_TRACE */
> > +
> >  #endif /* __LINUX_RCUTINY_H */
> > diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
> > index 226169d..c605b41 100644
> > --- a/include/linux/rcutree.h
> > +++ b/include/linux/rcutree.h
> > @@ -90,4 +90,6 @@ extern void exit_rcu(void);
> >  extern void rcu_scheduler_starting(void);
> >  extern int rcu_scheduler_active __read_mostly;
> >  
> > +extern bool rcu_watching_this_cpu(void);
> > +
> >  #endif /* __LINUX_RCUTREE_H */
> > diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
> > index 7e3b0d6..b14701f 100644
> > --- a/kernel/rcutiny.c
> > +++ b/kernel/rcutiny.c
> > @@ -176,7 +176,7 @@ void rcu_irq_enter(void)
> >  }
> >  EXPORT_SYMBOL_GPL(rcu_irq_enter);
> >  
> > -#ifdef CONFIG_DEBUG_LOCK_ALLOC
> > +#if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE)
> >  
> >  /*
> >   * Test whether RCU thinks that the current CPU is idle.
> > @@ -187,7 +187,7 @@ int rcu_is_cpu_idle(void)
> >  }
> >  EXPORT_SYMBOL(rcu_is_cpu_idle);
> >  
> > -#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
> > +#endif /* defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) */
> >  
> >  /*
> >   * Test whether the current CPU was interrupted from idle.  Nested
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index a06d172..38c6883 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -666,6 +666,19 @@ int rcu_is_cpu_idle(void)
> >  }
> >  EXPORT_SYMBOL(rcu_is_cpu_idle);
> >  
> > +/**
> > + * rcu_watching_this_cpu - are RCU read-side critical sections safe?
> > + *
> > + * Return true if RCU is watching the running CPU, which means that
> > + * this CPU can safely enter RCU read-side critical sections.  Unlike
> > + * rcu_is_cpu_idle(), the caller of rcu_watching_this_cpu() must have at
> > + * least disabled preemption.
> > + */
> > +bool rcu_watching_this_cpu(void)
> > +{
> > +	return !!__this_cpu_read(rcu_dynticks.dynticks_nesting);
> > +}
> > +
> >  #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU)
> >  
> >  /*
> > 
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
> 

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