Re: [PATCH 3/3] mutex: Add missing annotations

[Peter Zijlstra]

On Fri, Jan 24, 2020 at 08:12:20PM +0000, Jules Irenge wrote:
> Sparse reports false warnings and hide real warnings
> where mutex_lock() and mutex_unlock() are used within the kernel
> An example is within the kernel cgroup files
> where the below warnings are found
> |warning: context imbalance in cgroup_lock_and_drain_offline()
> | - wrong count at exit
> |warning: context imbalance in cgroup_procs_write_finish()
> |- wrong count at exit
> |warning: context imbalance in cgroup_procs_write_start()
> |- wrong count at exit.
> 
> To fix these,
> an __acquires(lock) is added to mutex_lock() declaration
> a __releases(lock) to mutex_unlock() declaration
> 
> Signed-off-by: Jules Irenge <jbi.octave@xxxxxxxxx>
> ---
>  include/linux/mutex.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mutex.h b/include/linux/mutex.h
> index aca8f36dfac9..a8ab4029913e 100644
> --- a/include/linux/mutex.h
> +++ b/include/linux/mutex.h
> @@ -162,7 +162,7 @@ do {									\
>  } while (0)
>  
>  #else
> -extern void mutex_lock(struct mutex *lock);
> +extern void mutex_lock(struct mutex *lock) __acquires(lock);
>  extern int __must_check mutex_lock_interruptible(struct mutex *lock);
>  extern int __must_check mutex_lock_killable(struct mutex *lock);
>  extern void mutex_lock_io(struct mutex *lock);
> @@ -181,7 +181,7 @@ extern void mutex_lock_io(struct mutex *lock);
>   * Returns 1 if the mutex has been acquired successfully, and 0 on contention.
>   */
>  extern int mutex_trylock(struct mutex *lock);
> -extern void mutex_unlock(struct mutex *lock);
> +extern void mutex_unlock(struct mutex *lock) __releases(lock);
>  
>  extern int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);

*groan*, I despise these sparse things.

The proposed patch only annotates a tiny part of the mutex interface,
and will thereby generate a flood of new (pointless) warnings. Worse,
annotating them all properly will require that __cond_lock() trainwreck.