Re: [PATCH RFC 08/24] lockdep: Annotate lockdep assertions for capability analysis

[Bart Van Assche <bvanassche@acm.org>]

On 2/6/25 10:10 AM, Marco Elver wrote:
> diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
> index 67964dc4db95..5cea929b2219 100644
> --- a/include/linux/lockdep.h
> +++ b/include/linux/lockdep.h
> @@ -282,16 +282,16 @@ extern void lock_unpin_lock(struct lockdep_map *lock, struct pin_cookie);
>   	do { WARN_ON_ONCE(debug_locks && !(cond)); } while (0)
>   
>   #define lockdep_assert_held(l)		\
> -	lockdep_assert(lockdep_is_held(l) != LOCK_STATE_NOT_HELD)
> +	do { lockdep_assert(lockdep_is_held(l) != LOCK_STATE_NOT_HELD); __assert_cap(l); } while (0)
>   
>   #define lockdep_assert_not_held(l)	\
>   	lockdep_assert(lockdep_is_held(l) != LOCK_STATE_HELD)
>   
>   #define lockdep_assert_held_write(l)	\
> -	lockdep_assert(lockdep_is_held_type(l, 0))
> +	do { lockdep_assert(lockdep_is_held_type(l, 0)); __assert_cap(l); } while (0)
>   
>   #define lockdep_assert_held_read(l)	\
> -	lockdep_assert(lockdep_is_held_type(l, 1))
> +	do { lockdep_assert(lockdep_is_held_type(l, 1)); __assert_shared_cap(l); } while (0)

These changes look wrong to me. The current behavior of
lockdep_assert_held(lock) is that it issues a kernel warning at
runtime if `lock` is not held when a lockdep_assert_held()
statement is executed. __assert_cap(lock) tells the compiler to
*ignore* the absence of __must_hold(lock). I think this is wrong.
The compiler should complain if a __must_hold(lock) annotation is
missing. While sparse does not support interprocedural analysis for
lock contexts, the Clang thread-safety checker supports this. If
function declarations are annotated with __must_hold(lock), Clang will
complain if the caller does not hold `lock`.

In other words, the above changes disable a useful compile-time check.
I think that useful compile-time checks should not be disabled.

Bart.