[PATCH v2] drm: fix drmm_mutex_init()

[Boris Brezillon]

On Fri, 19 May 2023 10:07:33 +0100
Matthew Auld <matthew.auld at intel.com> wrote:

> In mutex_init() lockdep identifies a lock by defining a special static
> key for each lock class. However if we wrap the macro in a function,
> like in drmm_mutex_init(), we end up generating:
> 
> int drmm_mutex_init(struct drm_device *dev, struct mutex *lock)
> {
>       static struct lock_class_key __key;
> 
>       __mutex_init((lock), "lock", &__key);
>       ....
> }
> 
> The static __key here is what lockdep uses to identify the lock class,
> however since this is just a normal function the key here will be
> created once, where all callers then use the same key. In effect the
> mutex->depmap.key will be the same pointer for different
> drmm_mutex_init() callers. This then results in impossible lockdep
> splats since lockdep thinks completely unrelated locks are the same lock
> class.
> 
> To fix this turn drmm_mutex_init() into a macro such that it generates a
> different "static struct lock_class_key __key" for each invocation,
> which looks to be inline with what mutex_init() wants.
> 
> v2:
>   - Revamp the commit message with clearer explanation of the issue.
>   - Rather export __drmm_mutex_release() than static inline.
> 
> Reported-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>
> Reported-by: Sarah Walker <sarah.walker at imgtec.com>
> Fixes: e13f13e039dc ("drm: Add DRM-managed mutex_init()")
> Cc: Stanislaw Gruszka <stanislaw.gruszka at linux.intel.com>
> Cc: Boris Brezillon <boris.brezillon at collabora.com>

Reviewed-by: Boris Brezillon <boris.brezillon at collabora.com>

> Cc: Thomas Zimmermann <tzimmermann at suse.de>
> Cc: Jocelyn Falempe <jfalempe at redhat.com>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: dri-devel at lists.freedesktop.org
> Signed-off-by: Matthew Auld <matthew.auld at intel.com>
> ---
>  drivers/gpu/drm/drm_managed.c | 22 ++--------------------
>  include/drm/drm_managed.h     | 18 +++++++++++++++++-
>  2 files changed, 19 insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_managed.c b/drivers/gpu/drm/drm_managed.c
> index 4cf214de50c4..c21c3f623033 100644
> --- a/drivers/gpu/drm/drm_managed.c
> +++ b/drivers/gpu/drm/drm_managed.c
> @@ -264,28 +264,10 @@ void drmm_kfree(struct drm_device *dev, void *data)
>  }
>  EXPORT_SYMBOL(drmm_kfree);
>  
> -static void drmm_mutex_release(struct drm_device *dev, void *res)
> +void __drmm_mutex_release(struct drm_device *dev, void *res)
>  {
>  	struct mutex *lock = res;
>  
>  	mutex_destroy(lock);
>  }
> -
> -/**
> - * drmm_mutex_init - &drm_device-managed mutex_init()
> - * @dev: DRM device
> - * @lock: lock to be initialized
> - *
> - * Returns:
> - * 0 on success, or a negative errno code otherwise.
> - *
> - * This is a &drm_device-managed version of mutex_init(). The initialized
> - * lock is automatically destroyed on the final drm_dev_put().
> - */
> -int drmm_mutex_init(struct drm_device *dev, struct mutex *lock)
> -{
> -	mutex_init(lock);
> -
> -	return drmm_add_action_or_reset(dev, drmm_mutex_release, lock);
> -}
> -EXPORT_SYMBOL(drmm_mutex_init);
> +EXPORT_SYMBOL(__drmm_mutex_release);
> diff --git a/include/drm/drm_managed.h b/include/drm/drm_managed.h
> index 359883942612..ad08f834af40 100644
> --- a/include/drm/drm_managed.h
> +++ b/include/drm/drm_managed.h
> @@ -105,6 +105,22 @@ char *drmm_kstrdup(struct drm_device *dev, const char *s, gfp_t gfp);
>  
>  void drmm_kfree(struct drm_device *dev, void *data);
>  
> -int drmm_mutex_init(struct drm_device *dev, struct mutex *lock);
> +void __drmm_mutex_release(struct drm_device *dev, void *res);
> +
> +/**
> + * drmm_mutex_init - &drm_device-managed mutex_init()
> + * @dev: DRM device
> + * @lock: lock to be initialized
> + *
> + * Returns:
> + * 0 on success, or a negative errno code otherwise.
> + *
> + * This is a &drm_device-managed version of mutex_init(). The initialized
> + * lock is automatically destroyed on the final drm_dev_put().
> + */
> +#define drmm_mutex_init(dev, lock) ({					     \
> +	mutex_init(lock);						     \
> +	drmm_add_action_or_reset(dev, __drmm_mutex_release, lock);	     \
> +})									     \
>  
>  #endif