drm: fix drmm_mutex_init()

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 <[e-mail]>
Reported-by: Sarah Walker <[e-mail]>
Fixes: e13f13e039dc ("drm: Add DRM-managed mutex_init()")
Cc: Stanislaw Gruszka <[e-mail]>
Cc: Boris Brezillon <[e-mail]>
Cc: Thomas Zimmermann <[e-mail]>
Cc: Jocelyn Falempe <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: [e-mail]
Signed-off-by: Matthew Auld <[e-mail]>
Reviewed-by: Boris Brezillon <[e-mail]>
Reviewed-by: Stanislaw Gruszka <[e-mail]>
Reviewed-by: Lucas De Marchi <[e-mail]>
Acked-by: Thomas Zimmermann <[e-mail]>
Signed-off-by: Thomas Zimmermann <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_managed.c

···
 }
 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);

include/drm/drm_managed.h

···
 
 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