Re: [RFC tip/locking/lockdep v6 19/20] rcu: Equip sleepable RCU with lockdep dependency graph checks

[Paul E. McKenney]

On Wed, Apr 11, 2018 at 09:56:44PM +0800, Boqun Feng wrote:
> Although all flavors of RCU are annotated correctly with lockdep
> annotations as recursive read locks, the 'check' parameter for their
> calls to lock_acquire() is unset. Which means RCU read locks are not
> added into the lockdep dependency graph. This is fine for all flavors
> except sleepable RCU, because the deadlock scenarios for them are
> simple: calling synchronize_rcu() and its friends inside their read-side
> critical sections. But for sleepable RCU, as there may be multiple
> instances with multiple classes, there are more deadlock cases.
> Considering the following:
> 
> 	TASK 1				TASK 2
> 	=======				========
> 	i = srcu_read_lock(&sa);	i = srcu_read_lock(&sb);
> 	synchronize_srcu(&sb);		synchronize_srcu(&sa);
> 	srcu_read_unlock(&sa);		srcu_read_unlock(&sb);
> 
> Neither TASK 1 or 2 could go out of the read-side critical sections,
> because they are waiting for each other at synchronize_srcu().
> 
> With the new improvement for lockdep, which allows us to detect
> deadlocks for recursive read locks, we can actually detect this. What we
> need to do are simply: a) mark srcu_read_{,un}lock() as 'check'
> lock_acquire() and b) annotate synchronize_srcu() as a empty
> grab-and-drop for a write lock (because synchronize_srcu() will wait for
> previous srcu_read_lock() to release, and won't block the next
> srcu_read_lock(), just like a empty write lock section).
> 
> This patch adds those to allow we check deadlocks related to sleepable
> RCU with lockdep.
> 
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>

Very cool!

One question though...  Won't this report a false-positive self-deadlock if
srcu_read_lock() is invoked from an interrupt handler?

							Thanx, Paul

> ---
>  include/linux/srcu.h  | 51 +++++++++++++++++++++++++++++++++++++++++++++++++--
>  kernel/rcu/srcutiny.c |  2 ++
>  kernel/rcu/srcutree.c |  2 ++
>  3 files changed, 53 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> index 33c1c698df09..23f397bd192c 100644
> --- a/include/linux/srcu.h
> +++ b/include/linux/srcu.h
> @@ -99,6 +99,49 @@ static inline int srcu_read_lock_held(const struct srcu_struct *sp)
>  	return lock_is_held(&sp->dep_map);
>  }
> 
> +/**
> + * lockdep annotations for srcu_read_{un,}lock, and synchronize_srcu():
> + *
> + * srcu_read_lock() and srcu_read_unlock() are similar to rcu_read_lock() and
> + * rcu_read_unlock(), they are recursive read locks. But we mark them as
> + * "check", they will be added into lockdep dependency graph for deadlock
> + * detection. And we also annotate synchronize_srcu() as a
> + * write_lock()+write_unlock(), because synchronize_srcu() will wait for any
> + * corresponding previous srcu_read_lock() to release, and that acts like a
> + * empty grab-and-drop write lock.
> + *
> + * We do so because multiple sleepable rcu instances may cause deadlock as
> + * follow:
> + *
> + *   Task 1:
> + *     ia = srcu_read_lock(&srcu_A);
> + *     synchronize_srcu(&srcu_B);
> + *     srcu_read_unlock(&srcu_A, ia);
> + *
> + *   Task 2:
> + *     ib = srcu_read_lock(&srcu_B);
> + *     synchronize_srcu(&srcu_A);
> + *     srcu_read_unlock(&srcu_B, ib);
> + *
> + * And we want lockdep to detect this or more complicated deadlock with SRCU
> + * involved.
> + */
> +static inline void srcu_lock_acquire(struct lockdep_map *map)
> +{
> +	lock_map_acquire_read(map);
> +}
> +
> +static inline void srcu_lock_release(struct lockdep_map *map)
> +{
> +	lock_map_release(map);
> +}
> +
> +static inline void srcu_lock_sync(struct lockdep_map *map)
> +{
> +	lock_map_acquire(map);
> +	lock_map_release(map);
> +}
> +
>  #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
> 
>  static inline int srcu_read_lock_held(const struct srcu_struct *sp)
> @@ -106,6 +149,10 @@ static inline int srcu_read_lock_held(const struct srcu_struct *sp)
>  	return 1;
>  }
> 
> +#define srcu_lock_acquire(m)	do { } while (0)
> +#define srcu_lock_release(m)	do { } while (0)
> +#define srcu_lock_sync(m)	do { } while (0)
> +
>  #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
> 
>  /**
> @@ -157,7 +204,7 @@ static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp)
>  	int retval;
> 
>  	retval = __srcu_read_lock(sp);
> -	rcu_lock_acquire(&(sp)->dep_map);
> +	srcu_lock_acquire(&(sp)->dep_map);
>  	return retval;
>  }
> 
> @@ -171,7 +218,7 @@ static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp)
>  static inline void srcu_read_unlock(struct srcu_struct *sp, int idx)
>  	__releases(sp)
>  {
> -	rcu_lock_release(&(sp)->dep_map);
> +	srcu_lock_release(&(sp)->dep_map);
>  	__srcu_read_unlock(sp, idx);
>  }
> 
> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> index 76ac5f50b2c7..bc89cb48d800 100644
> --- a/kernel/rcu/srcutiny.c
> +++ b/kernel/rcu/srcutiny.c
> @@ -188,6 +188,8 @@ void synchronize_srcu(struct srcu_struct *sp)
>  {
>  	struct rcu_synchronize rs;
> 
> +	srcu_lock_sync(&sp->dep_map);
> +
>  	init_rcu_head_on_stack(&rs.head);
>  	init_completion(&rs.completion);
>  	call_srcu(sp, &rs.head, wakeme_after_rcu);
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index d5cea81378cc..e2628e9275b9 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -997,6 +997,8 @@ EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
>   */
>  void synchronize_srcu(struct srcu_struct *sp)
>  {
> +	srcu_lock_sync(&sp->dep_map);
> +
>  	if (srcu_might_be_idle(sp) || rcu_gp_is_expedited())
>  		synchronize_srcu_expedited(sp);
>  	else
> -- 
> 2.16.2
>