Re: Should list_entry_rcu use rcu_dereference ?

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

On Fri, May 24, 2019 at 12:50:09PM -0400, Joel Fernandes wrote:
> On Fri, May 24, 2019 at 04:07:37AM -0400, Joel Fernandes wrote:
> [snip]
> > > > The purpose of this is to
> > > > check if any RCU reader is active at all. I believe after the flavor
> > > > consolidation effort, this is a sufficient condition from an RCU perspective.
> > > > Having some lockdep checking is better than no lockdep checking so I think it
> > > > is good to have. Let me know what you think about the below patch and I can
> > > > roll into a proper patch and send it as well (with proper comments).
> > > > 
> > > > I was able to trigger the lockdep check by removing the preempt_disable()
> > > > calls in ftrace_mod_get_kallsym() and insert some modules (PROVE_LOCKING and
> > > > FUNCION_TRACE enabled).
> > > 
> > > The big question is "what would use these?".  Although it would be good
> > > to replace uses of rcu_dereference_raw(), many of those are driven by a
> > > desire to support both RCU and non-RCU use cases, where in the non-RCU
> > > variant the user supplies the lock.
> > > 
> > > So would these actually see enough use to make their addition worthwhile?
> > > If so, which uses are you thinking in terms of?
> > 
> > Yes true. Actually my initial intent was to add some lockdep checking for
> > RCU-list traversal via list_for_each_entry_rcu(). Since the pointers are
> > traversed without any lockdep checking due to not going through the
> > rcu_dereference API family.
> > 
> > You are right the patch I shared is complicated, and I could just keep the
> > rcu_read_lock_any_held() and use that in list_for_each_entry_rcu for that
> > matter (and in any other list RCU APIs doing list traversals). In fact I
> > could probably also call this new API as: lockdep_assert_rcu_held().
> > 
> > Also I noticed there is a rcu_lockdep_assert() mentioned in the
> > Requirements.html part of the documentation, but didn't find such a
> > definition in the kernel sources. So we should probably also update that ;-)
> 
> And I see rcu_lockdep_assert() got converted to RCU_LOCKDEP_WARN in
> https://lore.kernel.org/patchwork/patch/580641/
> 
> I will toy with the idea of a lockdep_assert_rcu_held() which checks for
> consolidated RCU reader sections and use that for list RCU and such, and we
> can discuss more on the subsequent RFC.

Interesting...

RCU is now out of sync with the other lockdep assertions, but at Ingo
Molnar's request a few years back.  ;-)

							Thanx, Paul

> thanks!
> 
> 
> > 
> > From Requirements.html:
> > 	A given function might wish to check for RCU-related preconditions
> > 	upon entry, before using any other RCU API.
> > 	The <tt>rcu_lockdep_assert()</tt> does this job,
> > 	asserting the expression in kernels having lockdep enabled
> > 	and doing nothing otherwise.
> > 
> > > Sorry to be so skeptical, but it has not been that long since Linus
> > > read me the riot act over RCU complexity.  ;-)
> > 
> > No problem, perfectly legit skepticism ;-)
> > 
> > thanks,
> > 
> >  - Joel
> > 
> > > 
> > > 							Thanx, Paul
> > > 
> > > > ---8<-----------------------
> > > > 
> > > > diff --git a/include/linux/rculist.h b/include/linux/rculist.h
> > > > index e91ec9ddcd30..334c625ef421 100644
> > > > --- a/include/linux/rculist.h
> > > > +++ b/include/linux/rculist.h
> > > > @@ -273,9 +273,12 @@ static inline void list_splice_tail_init_rcu(struct list_head *list,
> > > >   *
> > > >   * This primitive may safely run concurrently with the _rcu list-mutation
> > > >   * primitives such as list_add_rcu() as long as it's guarded by rcu_read_lock().
> > > > + *
> > > > + * Use rcu_dereference_any() to ensure we are generically within an RCU reader
> > > > + * section (whether sched, bh or regular).
> > > >   */
> > > >  #define list_entry_rcu(ptr, type, member) \
> > > > -	container_of(READ_ONCE(ptr), type, member)
> > > > +	container_of(rcu_dereference_any(ptr), type, member)
> > > >  
> > > >  /*
> > > >   * Where are list_empty_rcu() and list_first_entry_rcu()?
> > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > > > index 922bb6848813..7481d93ed9bb 100644
> > > > --- a/include/linux/rcupdate.h
> > > > +++ b/include/linux/rcupdate.h
> > > > @@ -223,6 +223,7 @@ int debug_lockdep_rcu_enabled(void);
> > > >  int rcu_read_lock_held(void);
> > > >  int rcu_read_lock_bh_held(void);
> > > >  int rcu_read_lock_sched_held(void);
> > > > +int rcu_read_lock_any_held(void);
> > > >  
> > > >  #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
> > > >  
> > > > @@ -243,6 +244,12 @@ static inline int rcu_read_lock_sched_held(void)
> > > >  {
> > > >  	return !preemptible();
> > > >  }
> > > > +
> > > > +static inline int rcu_read_lock_any_held(void)
> > > > +{
> > > > +	return !preemptible();
> > > > +}
> > > > +
> > > >  #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
> > > >  
> > > >  #ifdef CONFIG_PROVE_RCU
> > > > @@ -472,6 +479,10 @@ static inline void rcu_preempt_sleep_check(void) { }
> > > >  	__rcu_dereference_check((p), (c) || rcu_read_lock_sched_held(), \
> > > >  				__rcu)
> > > >  
> > > > +#define rcu_dereference_any_check(p, c) \
> > > > +	__rcu_dereference_check((p), (c) || rcu_read_lock_any_held(), \
> > > > +				__rcu)
> > > > +
> > > >  /*
> > > >   * The tracing infrastructure traces RCU (we want that), but unfortunately
> > > >   * some of the RCU checks causes tracing to lock up the system.
> > > > @@ -525,6 +536,8 @@ static inline void rcu_preempt_sleep_check(void) { }
> > > >   */
> > > >  #define rcu_dereference_sched(p) rcu_dereference_sched_check(p, 0)
> > > >  
> > > > +#define rcu_dereference_any(p) rcu_dereference_any_check(p, 0)
> > > > +
> > > >  /**
> > > >   * rcu_pointer_handoff() - Hand off a pointer from RCU to other mechanism
> > > >   * @p: The pointer to hand off
> > > > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> > > > index c3bf44ba42e5..2dab75d34806 100644
> > > > --- a/kernel/rcu/update.c
> > > > +++ b/kernel/rcu/update.c
> > > > @@ -298,6 +298,31 @@ int rcu_read_lock_bh_held(void)
> > > >  }
> > > >  EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held);
> > > >  
> > > > +int rcu_read_lock_any_held(void)
> > > > +{
> > > > +	int lockdep_opinion = 0;
> > > > +
> > > > +	if (!debug_lockdep_rcu_enabled())
> > > > +		return 1;
> > > > +	if (!rcu_is_watching())
> > > > +		return 0;
> > > > +	if (!rcu_lockdep_current_cpu_online())
> > > > +		return 0;
> > > > +
> > > > +	if (lock_is_held(&rcu_lock_map))
> > > > +		return 1;
> > > > +
> > > > +	/* BH flavor */
> > > > +	if (in_softirq() || irqs_disabled())
> > > > +		return 1;
> > > > +
> > > > +	/* Sched flavor */
> > > > +	if (debug_locks)
> > > > +		lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
> > > > +	return lockdep_opinion || !preemptible();
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(rcu_read_lock_any_held);
> > > > +
> > > >  #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
> > > >  
> > > >  /**
> > > > -- 
> > > > 2.21.0.1020.gf2820cf01a-goog
> > > > 
>