Subject: Re: [PATCH 2/9] rcu: Add support for consolidated-RCU reader checking (v3)

From: "Paul E. McKenney" <[email-masked]>
Date: Tue, 16 Jul 2019 11:53:03 -0700

On Tue, Jul 16, 2019 at 02:46:49PM -0400, Joel Fernandes wrote:
> On Tue, Jul 16, 2019 at 11:38:33AM -0700, Paul E. McKenney wrote:
> > On Mon, Jul 15, 2019 at 10:36:58AM -0400, Joel Fernandes (Google) wrote:
> > > This patch adds support for checking RCU reader sections in list
> > > traversal macros. Optionally, if the list macro is called under SRCU or
> > > other lock/mutex protection, then appropriate lockdep expressions can be
> > > passed to make the checks pass.
> > > 
> > > Existing list_for_each_entry_rcu() invocations don't need to pass the
> > > optional fourth argument (cond) unless they are under some non-RCU
> > > protection and needs to make lockdep check pass.
> > > 
> > > Signed-off-by: Joel Fernandes (Google) <[email-masked]>
> > 
> > Now that I am on the correct version, again please fold in the checks
> > for the extra argument.  The ability to have an optional argument looks
> > quite helpful, especially when compared to growing the RCU API!
> 
> I did fold this and replied with a pull request URL based on /dev branch. But
> we can hold off on the pull requests until we decide on the below comments:
> 
> > A few more things below.
> > > ---
> > >  include/linux/rculist.h  | 28 ++++++++++++++++++++-----
> > >  include/linux/rcupdate.h |  7 +++++++
> > >  kernel/rcu/Kconfig.debug | 11 ++++++++++
> > >  kernel/rcu/update.c      | 44 ++++++++++++++++++++++++----------------
> > >  4 files changed, 67 insertions(+), 23 deletions(-)
> > > 
> > > diff --git a/include/linux/rculist.h b/include/linux/rculist.h
> > > index e91ec9ddcd30..1048160625bb 100644
> > > --- a/include/linux/rculist.h
> > > +++ b/include/linux/rculist.h
> > > @@ -40,6 +40,20 @@ static inline void INIT_LIST_HEAD_RCU(struct list_head *list)
> > >   */
> > >  #define list_next_rcu(list)	(*((struct list_head __rcu **)(&(list)->next)))
> > >  
> > > +/*
> > > + * Check during list traversal that we are within an RCU reader
> > > + */
> > > +
> > > +#ifdef CONFIG_PROVE_RCU_LIST
> > 
> > This new Kconfig option is OK temporarily, but unless there is reason to
> > fear malfunction that a few weeks of rcutorture, 0day, and -next won't
> > find, it would be better to just use CONFIG_PROVE_RCU.  The overall goal
> > is to reduce the number of RCU knobs rather than grow them, must though
> > history might lead one to believe otherwise.  :-/
> 
> If you want, we can try to drop this option and just use PROVE_RCU however I
> must say there may be several warnings that need to be fixed in a short
> period of time (even a few weeks may be too short) considering the 1000+
> uses of RCU lists.

Do many people other than me build with CONFIG_PROVE_RCU?  If so, then
that would be a good reason for a temporary CONFIG_PROVE_RCU_LIST,
as in going away in a release or two once the warnings get fixed.

> But I don't mind dropping it and it may just accelerate the fixing up of all
> callers.

I will let you decide based on the above question.  But if you have
CONFIG_PROVE_RCU_LIST, as noted below, it needs to depend on RCU_EXPERT.

							Thanx, Paul

> > > +#define __list_check_rcu(dummy, cond, ...)				\
> > > +	({								\
> > > +	RCU_LOCKDEP_WARN(!cond && !rcu_read_lock_any_held(),		\
> > > +			 "RCU-list traversed in non-reader section!");	\
> > > +	 })
> > > +#else
> > > +#define __list_check_rcu(dummy, cond, ...) ({})
> > > +#endif
> > > +
> > >  /*
> > >   * Insert a new entry between two known consecutive entries.
> > >   *
> > > @@ -343,14 +357,16 @@ static inline void list_splice_tail_init_rcu(struct list_head *list,
> > >   * @pos:	the type * to use as a loop cursor.
> > >   * @head:	the head for your list.
> > >   * @member:	the name of the list_head within the struct.
> > > + * @cond:	optional lockdep expression if called from non-RCU protection.
> > >   *
> > >   * This list-traversal primitive may safely run concurrently with
> > >   * the _rcu list-mutation primitives such as list_add_rcu()
> > >   * as long as the traversal is guarded by rcu_read_lock().
> > >   */
> > > -#define list_for_each_entry_rcu(pos, head, member) \
> > > -	for (pos = list_entry_rcu((head)->next, typeof(*pos), member); \
> > > -		&pos->member != (head); \
> > > +#define list_for_each_entry_rcu(pos, head, member, cond...)		\
> > > +	for (__list_check_rcu(dummy, ## cond, 0),			\
> > > +	     pos = list_entry_rcu((head)->next, typeof(*pos), member);	\
> > > +		&pos->member != (head);					\
> > >  		pos = list_entry_rcu(pos->member.next, typeof(*pos), member))
> > >  
> > >  /**
> > > @@ -616,13 +632,15 @@ static inline void hlist_add_behind_rcu(struct hlist_node *n,
> > >   * @pos:	the type * to use as a loop cursor.
> > >   * @head:	the head for your list.
> > >   * @member:	the name of the hlist_node within the struct.
> > > + * @cond:	optional lockdep expression if called from non-RCU protection.
> > >   *
> > >   * This list-traversal primitive may safely run concurrently with
> > >   * the _rcu list-mutation primitives such as hlist_add_head_rcu()
> > >   * as long as the traversal is guarded by rcu_read_lock().
> > >   */
> > > -#define hlist_for_each_entry_rcu(pos, head, member)			\
> > > -	for (pos = hlist_entry_safe (rcu_dereference_raw(hlist_first_rcu(head)),\
> > > +#define hlist_for_each_entry_rcu(pos, head, member, cond...)		\
> > > +	for (__list_check_rcu(dummy, ## cond, 0),			\
> > > +	     pos = hlist_entry_safe (rcu_dereference_raw(hlist_first_rcu(head)),\
> > >  			typeof(*(pos)), member);			\
> > >  		pos;							\
> > >  		pos = hlist_entry_safe(rcu_dereference_raw(hlist_next_rcu(\
> > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > > index 8f7167478c1d..f3c29efdf19a 100644
> > > --- a/include/linux/rcupdate.h
> > > +++ b/include/linux/rcupdate.h
> > > @@ -221,6 +221,7 @@ 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 */
> > >  
> > > @@ -241,6 +242,12 @@ 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
> > > diff --git a/kernel/rcu/Kconfig.debug b/kernel/rcu/Kconfig.debug
> > > index 5ec3ea4028e2..7fbd21dbfcd0 100644
> > > --- a/kernel/rcu/Kconfig.debug
> > > +++ b/kernel/rcu/Kconfig.debug
> > > @@ -8,6 +8,17 @@ menu "RCU Debugging"
> > >  config PROVE_RCU
> > >  	def_bool PROVE_LOCKING
> > >  
> > > +config PROVE_RCU_LIST
> > > +	bool "RCU list lockdep debugging"
> > > +	depends on PROVE_RCU
> > 
> > This must also depend on RCU_EXPERT.  
> 
> Sure.
> 
> > > +	default n
> > > +	help
> > > +	  Enable RCU lockdep checking for list usages. By default it is
> > > +	  turned off since there are several list RCU users that still
> > > +	  need to be converted to pass a lockdep expression. To prevent
> > > +	  false-positive splats, we keep it default disabled but once all
> > > +	  users are converted, we can remove this config option.
> > > +
> > >  config TORTURE_TEST
> > >  	tristate
> > >  	default n
> > > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> > > index 9dd5aeef6e70..b7a4e3b5fa98 100644
> > > --- a/kernel/rcu/update.c
> > > +++ b/kernel/rcu/update.c
> > > @@ -91,14 +91,18 @@ module_param(rcu_normal_after_boot, int, 0);
> > >   * Similarly, we avoid claiming an SRCU read lock held if the current
> > >   * CPU is offline.
> > >   */
> > > +#define rcu_read_lock_held_common()		\
> > > +	if (!debug_lockdep_rcu_enabled())	\
> > > +		return 1;			\
> > > +	if (!rcu_is_watching())			\
> > > +		return 0;			\
> > > +	if (!rcu_lockdep_current_cpu_online())	\
> > > +		return 0;
> > 
> > Nice abstraction of common code!
> 
> Thanks!
>