[PATCH v2] drm: fix drmm_mutex_init()

[Matthew Auld]

On 22/05/2023 10:43, Thomas Zimmermann wrote:
> Hi
> 
> Am 19.05.23 um 11:07 schrieb Matthew Auld:
>> 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>
> 
> Acked-by: Thomas Zimmermann <tzimmermann at suse.de>
> 
> Shall I add the patch to drm-misc-fixes?

Yes, please do. Thanks.

> 
> Best regards
> Thomas
> 
>> ---
>>   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
>