[Intel-gfx] [PATCH] drm: Make each driver's struct_mutex its own subclass

[Chris Wilson]

On Sat, Dec 10, 2016 at 09:28:02PM +0000, Chris Wilson wrote:
> On Sat, Dec 10, 2016 at 09:23:35PM +0000, Chris Wilson wrote:
> > On Sat, Dec 10, 2016 at 10:19:30PM +0100, Daniel Vetter wrote:
> > > On Fri, Dec 09, 2016 at 04:52:32PM +0000, Chris Wilson wrote:
> > > > With prime, we are running into false circular dependencies based on the
> > > > order in which two drivers may lock their own struct_mutex wrt to a
> > > > common lock (like the reservation->lock). Work around this by adding the
> > > > lock_class_key to the struct drm_driver such that each driver can have
> > > > its own subclass of struct_mutex. Circular dependencies between drivers
> > > > will now be ignored, but real circular dependencies on any one mutex
> > > > will still be caught. A driver creating more than one device will still
> > > > need to be careful!
> > > > 
> > > > Reported-by: Tobias Klausmann <tobias.johannes.klausmann at mni.thm.de>
> > > > Reported-by: Hans de Goede <hdegoede at redhat.com>
> > > > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > > 
> > > Where does this even happen? i915, msm and udl are the only drivers left
> > > over that do struct_mutex, and i915 can't really share buffers with msm,
> > > and udl doesn't do reservations. How exactly does this still go boom in
> > > latest upstream?
> > 
> > How about cc: stable?
> > 
> > The reports are nouveau vs i915. I was quite pleased with the
> > drm_driver_class!
> 
> Ah, you may have removed any direct calls to struct_mutex from nouveau,
> but it is still using struct_mutex around its GEM bo references.
> 
> git grep drm_gem_object_unreference_unlocked -- drivers/gpu/drm/nouveau/ | wc -l
> 13

Either s/drm_gem_object_unreference_unlocked/__drm_gem_object_unreference/

Or

diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index 465bacd0a630..824a7780de06 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -826,11 +826,13 @@ drm_gem_object_unreference_unlocked(struct drm_gem_object *obj)
                return;
 
        dev = obj->dev;
-       might_lock(&dev->struct_mutex);
-
-       if (dev->driver->gem_free_object_unlocked)
+       if (dev->driver->gem_free_object_unlocked) {
                kref_put(&obj->refcount, drm_gem_object_free);
-       else if (kref_put_mutex(&obj->refcount, drm_gem_object_free,
+               return;
+       }
+
+       might_lock(&dev->struct_mutex);
+       if (kref_put_mutex(&obj->refcount, drm_gem_object_free,
                                &dev->struct_mutex))
                mutex_unlock(&dev->struct_mutex);
 }

That's a false might_lock() that really should be pushed to kref_put_mutex()
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre