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 > > > > >
