Re: [Outreachy kernel] [PATCH] drm/gem: use new idr deletion interface to cleanup drm_gem_handle_delete()

[Aishwarya Pant <aishpant@gmail.com>]

On Tue, Sep 26, 2017 at 10:20:47AM +0200, Daniel Vetter wrote:
> On Tue, Sep 26, 2017 at 12:17:28AM +0530, Aishwarya Pant wrote:
> > The IDR deletion interface now returns the deleted entry or NULL if it was not
> > present. So we don't have to do the extra work of checking if we have a
> > reference on the drm_gem_object, this can be handled by checking the return
> > value from idr_remove() and the extra locks can be dropped.
> > 
> > Signed-off-by: Aishwarya Pant <aishpant@gmail.com>
> 
> Haneen is already working on this task, with the patch just merged. Please
> coordinate with mentors (me or Sean Paul) before starting on something to
> avoid such clashes in the future.

Apologies for the mix-up, I'll make sure to check in with the mentors before
starting a task.

I looked over the other patch sent by Haneen today, there is one thing that I
could use some clarity on. I'm not sure how the -this is gross- comment is
obsolete since we can drop the check to idr_replace() and use the return value
from idr_remove() which seems neater in my opinion.

> 
> Note also that some todo items are just ideas, and need confirmation with
> relevant maintainers first.
> 
> Thanks, Daniel
> > ---
> >  drivers/gpu/drm/drm_gem.c | 21 ++-------------------
> >  1 file changed, 2 insertions(+), 19 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> > index c55f338..f62757a 100644
> > --- a/drivers/gpu/drm/drm_gem.c
> > +++ b/drivers/gpu/drm/drm_gem.c
> > @@ -282,29 +282,12 @@ drm_gem_handle_delete(struct drm_file *filp, u32 handle)
> >  {
> >  	struct drm_gem_object *obj;
> >  
> > -	/* This is gross. The idr system doesn't let us try a delete and
> > -	 * return an error code.  It just spews if you fail at deleting.
> > -	 * So, we have to grab a lock around finding the object and then
> > -	 * doing the delete on it and dropping the refcount, or the user
> > -	 * could race us to double-decrement the refcount and cause a
> > -	 * use-after-free later.  Given the frequency of our handle lookups,
> > -	 * we may want to use ida for number allocation and a hash table
> > -	 * for the pointers, anyway.
> > -	 */
> >  	spin_lock(&filp->table_lock);
> > -
> > -	/* Check if we currently have a reference on the object */
> > -	obj = idr_replace(&filp->object_idr, NULL, handle);
> > -	spin_unlock(&filp->table_lock);
> > -	if (IS_ERR_OR_NULL(obj))
> > +	obj = idr_remove(&filp->object_idr, handle);
> > +	if (!obj)
> >  		return -EINVAL;
> > -
> >  	/* Release driver's reference and decrement refcount. */
> >  	drm_gem_object_release_handle(handle, obj, filp);
> > -
> > -	/* And finally make the handle available for future allocations. */
> > -	spin_lock(&filp->table_lock);
> > -	idr_remove(&filp->object_idr, handle);
> >  	spin_unlock(&filp->table_lock);
> >  
> >  	return 0;
> > -- 
> > 2.7.4
> > 
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> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
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