Re: [PATCH 09/35] drm/gem: support BO freeing without dev->struct_mutex

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On Wed, Apr 27, 2016 at 11:31:00AM +0200, Lucas Stach wrote:
> Am Dienstag, den 26.04.2016, 19:29 +0200 schrieb Daniel Vetter:
> > Finally all the core gem and a lot of drivers are entirely free of
> > dev->struct_mutex depencies, and we can start to have an entirely
> > lockless unref path.
> > 
> > To make sure that no one who touches the core code accidentally breaks
> > existing drivers which still require dev->struct_mutex I've made the
> > might_lock check unconditional.
> > 
> > While at it de-inline the ref/unref functions, they've become a bit
> > too big.
> > 
> > v2: Make it not leak like a sieve.
> > 
> > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> 
> Two mostly cosmetic comments below, otherwise looks good:
> 
> Reviewed-by: Lucas Stach <l.stach@xxxxxxxxxxxxxx>
> 
> > ---
> >  drivers/gpu/drm/drm_gem.c | 64 ++++++++++++++++++++++++++++++++++++++---------
> >  include/drm/drmP.h        | 12 ++++++++-
> >  include/drm/drm_gem.h     | 45 ++-------------------------------
> >  3 files changed, 65 insertions(+), 56 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> > index 25dac31eef37..8f2eff448bb5 100644
> > --- a/drivers/gpu/drm/drm_gem.c
> > +++ b/drivers/gpu/drm/drm_gem.c
> > @@ -788,16 +788,7 @@ drm_gem_object_release(struct drm_gem_object *obj)
> >  }
> >  EXPORT_SYMBOL(drm_gem_object_release);
> >  
> > -/**
> > - * drm_gem_object_free - free a GEM object
> > - * @kref: kref of the object to free
> > - *
> > - * Called after the last reference to the object has been lost.
> > - * Must be called holding struct_ mutex
> > - *
> > - * Frees the object
> > - */
> > -void
> > +static void
> >  drm_gem_object_free(struct kref *kref)
> >  {
> >  	struct drm_gem_object *obj =
> > @@ -806,10 +797,59 @@ drm_gem_object_free(struct kref *kref)
> >  
> >  	WARN_ON(!mutex_is_locked(&dev->struct_mutex));
> >  
> > -	if (dev->driver->gem_free_object != NULL)
> > +	if (dev->driver->gem_free_object_unlocked != NULL)
> 
> Why those explicit != NULL checks? The style mostly used in this file is
> to omit those, like "if (dev->driver->gem_free_object_unlocked)". Same
> comment applies to several hunks of this patch.

I move some of these functions around, the != NULL checks where there
already. I can easily change all the ones I touch.

> > +		dev->driver->gem_free_object_unlocked(obj);
> > +	else if (dev->driver->gem_free_object != NULL)
> >  		dev->driver->gem_free_object(obj);
> >  }
> > -EXPORT_SYMBOL(drm_gem_object_free);
> > +
> > +/**
> > + * drm_gem_object_unreference_unlocked - release a GEM BO reference
> > + * @obj: GEM buffer object
> > + *
> > + * This releases a reference to @obj. Callers must not hold the
> > + * dev->struct_mutex lock when calling this function.
> > + */
> > +void
> > +drm_gem_object_unreference_unlocked(struct drm_gem_object *obj)
> > +{
> > +	struct drm_device *dev;
> > +
> > +	if (!obj)
> > +		return;
> > +
> > +	dev = obj->dev;
> > +	might_lock(&dev->struct_mutex);
> > +
> > +	if (dev->driver->gem_free_object != NULL)
> > +		kref_put(&obj->refcount, drm_gem_object_free);
> > +	else if (kref_put_mutex(&obj->refcount, drm_gem_object_free,
> > +				&dev->struct_mutex))
> > +		mutex_unlock(&dev->struct_mutex);
> > +}
> > +EXPORT_SYMBOL(drm_gem_object_unreference_unlocked);
> > +
> > +/**
> > + * drm_gem_object_unreference - release a GEM BO reference
> > + * @obj: GEM buffer object
> > + *
> > + * This releases a reference to @obj. Callers must hold the dev->struct_mutex
> > + * lock when calling this function, even when the driver doesn't use
> > + * dev->struct_mutex for anything.
> > + *
> > + * For drivers not encumbered with legacy locking use
> > + * drm_gem_object_unreference_unlocked() instead.
> > + */
> > +void
> > +drm_gem_object_unreference(struct drm_gem_object *obj)
> > +{
> > +	if (obj != NULL) {
> > +		WARN_ON(!mutex_is_locked(&obj->dev->struct_mutex));
> > +
> > +		kref_put(&obj->refcount, drm_gem_object_free);
> > +	}
> > +}
> > +EXPORT_SYMBOL(drm_gem_object_unreference);
> >  
> >  /**
> >   * drm_gem_vm_open - vma->ops->open implementation for GEM
> > diff --git a/include/drm/drmP.h b/include/drm/drmP.h
> > index c81dd2250fc6..7e30b3d2b25c 100644
> > --- a/include/drm/drmP.h
> > +++ b/include/drm/drmP.h
> > @@ -583,9 +583,19 @@ struct drm_driver {
> >  	 * Driver-specific constructor for drm_gem_objects, to set up
> >  	 * obj->driver_private.
> >  	 *
> > -	 * Returns 0 on success.
> > +	 * This is deprecated and should not be used by new drivers. Use
> > +	 * @gem_free_object_unlocked instead.
> >  	 */
> >  	void (*gem_free_object) (struct drm_gem_object *obj);
> > +
> > +	/**
> > +	 * Driver-specific constructor for drm_gem_objects, to set up
> > +	 * obj->driver_private.
> 
> This part of the comment is really off. I see it's just copy&paste from
> the comment above gem_free_object, but I think it should have been
> placed above gem_open_object and has nothing to do with the free*
> callbacks.

Oops, wanted to add proper kerneldoc for this new hook and the old one to
explain the differences, and totally failed at that. Will fix up.
-Daniel

> 
> >  This is for drivers which are not encumbered
> > +	 * with dev->struct_mutex legacy locking schemes. Use this hook instead
> > +	 * of @gem_free_object.
> > +	 */
> > +	void (*gem_free_object_unlocked) (struct drm_gem_object *obj);
> > +
> >  	int (*gem_open_object) (struct drm_gem_object *, struct drm_file *);
> >  	void (*gem_close_object) (struct drm_gem_object *, struct drm_file *);
> >  
> > diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
> > index 0b3e11ab8757..ae1c7f18eec0 100644
> > --- a/include/drm/drm_gem.h
> > +++ b/include/drm/drm_gem.h
> > @@ -175,7 +175,6 @@ struct drm_gem_object {
> >  };
> >  
> >  void drm_gem_object_release(struct drm_gem_object *obj);
> > -void drm_gem_object_free(struct kref *kref);
> >  int drm_gem_object_init(struct drm_device *dev,
> >  			struct drm_gem_object *obj, size_t size);
> >  void drm_gem_private_object_init(struct drm_device *dev,
> > @@ -199,48 +198,8 @@ drm_gem_object_reference(struct drm_gem_object *obj)
> >  	kref_get(&obj->refcount);
> >  }
> >  
> > -/**
> > - * drm_gem_object_unreference - release a GEM BO reference
> > - * @obj: GEM buffer object
> > - *
> > - * This releases a reference to @obj. Callers must hold the dev->struct_mutex
> > - * lock when calling this function, even when the driver doesn't use
> > - * dev->struct_mutex for anything.
> > - *
> > - * For drivers not encumbered with legacy locking use
> > - * drm_gem_object_unreference_unlocked() instead.
> > - */
> > -static inline void
> > -drm_gem_object_unreference(struct drm_gem_object *obj)
> > -{
> > -	if (obj != NULL) {
> > -		WARN_ON(!mutex_is_locked(&obj->dev->struct_mutex));
> > -
> > -		kref_put(&obj->refcount, drm_gem_object_free);
> > -	}
> > -}
> > -
> > -/**
> > - * drm_gem_object_unreference_unlocked - release a GEM BO reference
> > - * @obj: GEM buffer object
> > - *
> > - * This releases a reference to @obj. Callers must not hold the
> > - * dev->struct_mutex lock when calling this function.
> > - */
> > -static inline void
> > -drm_gem_object_unreference_unlocked(struct drm_gem_object *obj)
> > -{
> > -	struct drm_device *dev;
> > -
> > -	if (!obj)
> > -		return;
> > -
> > -	dev = obj->dev;
> > -	if (kref_put_mutex(&obj->refcount, drm_gem_object_free, &dev->struct_mutex))
> > -		mutex_unlock(&dev->struct_mutex);
> > -	else
> > -		might_lock(&dev->struct_mutex);
> > -}
> > +void drm_gem_object_unreference_unlocked(struct drm_gem_object *obj);
> > +void drm_gem_object_unreference(struct drm_gem_object *obj);
> >  
> >  int drm_gem_handle_create(struct drm_file *file_priv,
> >  			  struct drm_gem_object *obj,
> 
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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