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SubjectRe: [PATCH 2/9] rcu: Add support for consolidated-RCU reader checking (v3)
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) <joel@joelfernandes.org>
>
> 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.

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

> > +#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!

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