Re: [PATCH] lockdep: lockdep_set_notrack_class()

[Waiman Long <longman@redhat.com>]

On 6/30/24 01:10, Kent Overstreet wrote:
> Add a new helper to disable lockdep tracking entirely for a given class.
>
> This is needed for bcachefs, which takes too many btree node locks for
> lockdep to track. Instead, we have a single lockdep_map for "btree_trans
> has any btree nodes locked", which makes more since given that we have
> centralized lock management and a cycle detector.

Could you explain a bit more what the current novalidate_class is 
lacking WRT to the bcachefs lock? Is it excessive performance overhead 
or some bogus lockdep warning?

Cheers,
Longman

>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Waiman Long <longman@redhat.com>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
> ---
>   fs/bcachefs/btree_locking.c   | 2 +-
>   include/linux/lockdep.h       | 4 ++++
>   include/linux/lockdep_types.h | 1 +
>   kernel/locking/lockdep.c      | 9 ++++++++-
>   4 files changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/fs/bcachefs/btree_locking.c b/fs/bcachefs/btree_locking.c
> index 79057cde4c41..16834a6b4c3d 100644
> --- a/fs/bcachefs/btree_locking.c
> +++ b/fs/bcachefs/btree_locking.c
> @@ -10,7 +10,7 @@ void bch2_btree_lock_init(struct btree_bkey_cached_common *b,
>   			  enum six_lock_init_flags flags)
>   {
>   	__six_lock_init(&b->lock, "b->c.lock", &bch2_btree_node_lock_key, flags);
> -	lockdep_set_novalidate_class(&b->lock);
> +	lockdep_set_notrack_class(&b->lock);
>   }
>   
>   #ifdef CONFIG_LOCKDEP
> diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
> index 08b0d1d9d78b..b76f1bcd2f7f 100644
> --- a/include/linux/lockdep.h
> +++ b/include/linux/lockdep.h
> @@ -181,6 +181,9 @@ static inline void lockdep_init_map(struct lockdep_map *lock, const char *name,
>   #define lockdep_set_novalidate_class(lock) \
>   	lockdep_set_class_and_name(lock, &__lockdep_no_validate__, #lock)
>   
> +#define lockdep_set_notrack_class(lock) \
> +	lockdep_set_class_and_name(lock, &__lockdep_no_track__, #lock)
> +
>   /*
>    * Compare locking classes
>    */
> @@ -338,6 +341,7 @@ static inline void lockdep_set_selftest_task(struct task_struct *task)
>   #define lockdep_set_subclass(lock, sub)		do { } while (0)
>   
>   #define lockdep_set_novalidate_class(lock) do { } while (0)
> +#define lockdep_set_notrack_class(lock) do { } while (0)
>   
>   /*
>    * We don't define lockdep_match_class() and lockdep_match_key() for !LOCKDEP
> diff --git a/include/linux/lockdep_types.h b/include/linux/lockdep_types.h
> index 70d30d40ea4a..9f361d3ab9d9 100644
> --- a/include/linux/lockdep_types.h
> +++ b/include/linux/lockdep_types.h
> @@ -80,6 +80,7 @@ struct lock_class_key {
>   };
>   
>   extern struct lock_class_key __lockdep_no_validate__;
> +extern struct lock_class_key __lockdep_no_track__;
>   
>   struct lock_trace;
>   
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 151bd3de5936..b6bb9fcd992a 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -4918,6 +4918,9 @@ EXPORT_SYMBOL_GPL(lockdep_init_map_type);
>   struct lock_class_key __lockdep_no_validate__;
>   EXPORT_SYMBOL_GPL(__lockdep_no_validate__);
>   
> +struct lock_class_key __lockdep_no_track__;
> +EXPORT_SYMBOL_GPL(__lockdep_no_track__);
> +
>   #ifdef CONFIG_PROVE_LOCKING
>   void lockdep_set_lock_cmp_fn(struct lockdep_map *lock, lock_cmp_fn cmp_fn,
>   			     lock_print_fn print_fn)
> @@ -5002,6 +5005,9 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
>   	if (unlikely(!debug_locks))
>   		return 0;
>   
> +	if (unlikely(lock->key == &__lockdep_no_track__))
> +		return 0;
> +
>   	if (!prove_locking || lock->key == &__lockdep_no_validate__)
>   		check = 0;
>   
> @@ -5764,7 +5770,8 @@ void lock_release(struct lockdep_map *lock, unsigned long ip)
>   
>   	trace_lock_release(lock, ip);
>   
> -	if (unlikely(!lockdep_enabled()))
> +	if (unlikely(!lockdep_enabled() ||
> +		     lock->key == &__lockdep_no_track__))
>   		return;
>   
>   	raw_local_irq_save(flags);