[PATCH v2] drm: fix drmm_mutex_init()

[Stanislaw Gruszka]

On Fri, May 19, 2023 at 10:07:33AM +0100, Matthew Auld 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>
> 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>
Reviewed-by: Stanislaw Gruszka <stanislaw.gruszka at linux.intel.com>

Regards
Stanislaw