[Intel-xe] [PATCH v2] drm: fix drmm_mutex_init()

[Lucas De Marchi]

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>


that explains well some splats we've been seeing on xe.

Reviewed-by: Lucas De Marchi <lucas.demarchi at intel.com>

thanks
Lucas De Marchi

>---
> 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
>-- 
>2.40.1
>