Re: [PATCH net-next] rcu: prevent RCU_LOCKDEP_WARN() from swallowing the condition

["Paul E. McKenney" <paulmck@kernel.org>]

On Mon, Sep 14, 2020 at 03:47:38PM -0700, Jakub Kicinski wrote:
> On Mon, 14 Sep 2020 16:21:22 -0400 Joel Fernandes wrote:
> > On Tue, Sep 08, 2020 at 05:27:51PM -0700, Jakub Kicinski wrote:
> > > On Tue, 08 Sep 2020 21:15:56 +0300 nikolay@cumulusnetworks.com wrote:  
> > > > Ah, you want to solve it for all. :) 
> > > > Looks and sounds good to me, 
> > > > Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>  
> > > 
> > > Actually, I give up, lockdep_is_held() is not defined without
> > > CONFIG_LOCKDEP, let's just go with your patch..  
> > 
> > Care to send a patch just for the RCU macro then? Not sure what Dave is
> > applying but if the net-next tree is not taking the RCU macro change, then
> > send another one with my tag:
> 
> Seems like quite a few places depend on the macro disappearing its
> argument. I was concerned that it's going to be had to pick out whether
> !LOCKDEP builds should return true or false from LOCKDEP helpers, but
> perhaps relying on the linker errors even more is not such poor taste?
> 
> Does the patch below look acceptable to you?

The thing to check would be whether all compilers do sufficient
dead-code elimination (it used to be that they did not).  One way to
get a quick sniff test of this would be to make sure that a dead-code
lockdep_is_held() is in common code, and then expose this patch to kbuild
test robot.

Seem reasonable?

							Thanx, Paul

> --->8------------
> 
> rcu: prevent RCU_LOCKDEP_WARN() from swallowing the condition
> 
> We run into a unused variable warning in bridge code when
> variable is only used inside the condition of
> rcu_dereference_protected().
> 
>  #define mlock_dereference(X, br) \
> 	rcu_dereference_protected(X, lockdep_is_held(&br->multicast_lock))
> 
> Since on builds with CONFIG_PROVE_RCU=n rcu_dereference_protected()
> compiles to nothing the compiler doesn't see the variable use.
> 
> Prevent the warning by adding the condition as dead code.
> We need to un-hide the declaration of lockdep_tasklist_lock_is_held(),
> lockdep_sock_is_held(), RCU lock maps and remove some declarations
> in net/sched header, because they have a wrong type.
> 
> Add forward declarations of lockdep_is_held(), lock_is_held() which
> will cause a linker errors if actually used with !LOCKDEP.
> At least RCU expects some locks _not_ to be held so it's hard to
> pick true/false for a dummy implementation.
> 
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
> ---
>  include/linux/lockdep.h        |  6 ++++++
>  include/linux/rcupdate.h       | 11 ++++++-----
>  include/linux/rcupdate_trace.h |  4 ++--
>  include/linux/sched/task.h     |  2 --
>  include/net/sch_generic.h      | 12 ------------
>  include/net/sock.h             |  2 --
>  6 files changed, 14 insertions(+), 23 deletions(-)
> 
> diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
> index 6a584b3e5c74..c4b6225ee320 100644
> --- a/include/linux/lockdep.h
> +++ b/include/linux/lockdep.h
> @@ -371,6 +371,12 @@ static inline void lockdep_unregister_key(struct lock_class_key *key)
>  
>  #define lockdep_depth(tsk)	(0)
>  
> +/*
> + * Dummy forward declarations, allow users to write less ifdef-y code
> + * and depend on dead code elimination.
> + */
> +int lock_is_held(const void *);
> +int lockdep_is_held(const void *);
>  #define lockdep_is_held_type(l, r)		(1)
>  
>  #define lockdep_assert_held(l)			do { (void)(l); } while (0)
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index d15d46db61f7..50d45781fa99 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -234,6 +234,11 @@ bool rcu_lockdep_current_cpu_online(void);
>  static inline bool rcu_lockdep_current_cpu_online(void) { return true; }
>  #endif /* #else #if defined(CONFIG_HOTPLUG_CPU) && defined(CONFIG_PROVE_RCU) */
>  
> +extern struct lockdep_map rcu_lock_map;
> +extern struct lockdep_map rcu_bh_lock_map;
> +extern struct lockdep_map rcu_sched_lock_map;
> +extern struct lockdep_map rcu_callback_map;
> +
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
>  
>  static inline void rcu_lock_acquire(struct lockdep_map *map)
> @@ -246,10 +251,6 @@ static inline void rcu_lock_release(struct lockdep_map *map)
>  	lock_release(map, _THIS_IP_);
>  }
>  
> -extern struct lockdep_map rcu_lock_map;
> -extern struct lockdep_map rcu_bh_lock_map;
> -extern struct lockdep_map rcu_sched_lock_map;
> -extern struct lockdep_map rcu_callback_map;
>  int debug_lockdep_rcu_enabled(void);
>  int rcu_read_lock_held(void);
>  int rcu_read_lock_bh_held(void);
> @@ -320,7 +321,7 @@ static inline void rcu_preempt_sleep_check(void) { }
>  
>  #else /* #ifdef CONFIG_PROVE_RCU */
>  
> -#define RCU_LOCKDEP_WARN(c, s) do { } while (0)
> +#define RCU_LOCKDEP_WARN(c, s) do { } while (0 && (c))
>  #define rcu_sleep_check() do { } while (0)
>  
>  #endif /* #else #ifdef CONFIG_PROVE_RCU */
> diff --git a/include/linux/rcupdate_trace.h b/include/linux/rcupdate_trace.h
> index aaaac8ac927c..25cdef506cae 100644
> --- a/include/linux/rcupdate_trace.h
> +++ b/include/linux/rcupdate_trace.h
> @@ -11,10 +11,10 @@
>  #include <linux/sched.h>
>  #include <linux/rcupdate.h>
>  
> -#ifdef CONFIG_DEBUG_LOCK_ALLOC
> -
>  extern struct lockdep_map rcu_trace_lock_map;
>  
> +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> +
>  static inline int rcu_read_lock_trace_held(void)
>  {
>  	return lock_is_held(&rcu_trace_lock_map);
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index a98965007eef..9f943c391df9 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -47,9 +47,7 @@ extern spinlock_t mmlist_lock;
>  extern union thread_union init_thread_union;
>  extern struct task_struct init_task;
>  
> -#ifdef CONFIG_PROVE_RCU
>  extern int lockdep_tasklist_lock_is_held(void);
> -#endif /* #ifdef CONFIG_PROVE_RCU */
>  
>  extern asmlinkage void schedule_tail(struct task_struct *prev);
>  extern void init_idle(struct task_struct *idle, int cpu);
> diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
> index d60e7c39d60c..1aaa9e3d2e9c 100644
> --- a/include/net/sch_generic.h
> +++ b/include/net/sch_generic.h
> @@ -432,7 +432,6 @@ struct tcf_block {
>  	struct mutex proto_destroy_lock; /* Lock for proto_destroy hashtable. */
>  };
>  
> -#ifdef CONFIG_PROVE_LOCKING
>  static inline bool lockdep_tcf_chain_is_locked(struct tcf_chain *chain)
>  {
>  	return lockdep_is_held(&chain->filter_chain_lock);
> @@ -442,17 +441,6 @@ static inline bool lockdep_tcf_proto_is_locked(struct tcf_proto *tp)
>  {
>  	return lockdep_is_held(&tp->lock);
>  }
> -#else
> -static inline bool lockdep_tcf_chain_is_locked(struct tcf_block *chain)
> -{
> -	return true;
> -}
> -
> -static inline bool lockdep_tcf_proto_is_locked(struct tcf_proto *tp)
> -{
> -	return true;
> -}
> -#endif /* #ifdef CONFIG_PROVE_LOCKING */
>  
>  #define tcf_chain_dereference(p, chain)					\
>  	rcu_dereference_protected(p, lockdep_tcf_chain_is_locked(chain))
> diff --git a/include/net/sock.h b/include/net/sock.h
> index eaa5cac5e836..1c67b1297a72 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -1566,13 +1566,11 @@ do {									\
>  	lockdep_init_map(&(sk)->sk_lock.dep_map, (name), (key), 0);	\
>  } while (0)
>  
> -#ifdef CONFIG_LOCKDEP
>  static inline bool lockdep_sock_is_held(const struct sock *sk)
>  {
>  	return lockdep_is_held(&sk->sk_lock) ||
>  	       lockdep_is_held(&sk->sk_lock.slock);
>  }
> -#endif
>  
>  void lock_sock_nested(struct sock *sk, int subclass);
>  
> -- 
> 2.24.1
>