| Message ID | 20170715095328.25671-1-daniel.vetter@ffwll.ch (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
Quoting Daniel Vetter (2017-07-15 10:53:28) > For modern drivers the DRM core doesn't use struct_mutex at all, which > means it's defacto a driver-private lock. But since we still need it > for legacy drivers we can't initialize it in drivers, which means all > the different instances share one lockdep key. Despite that they might > be placed in totally different places in the locking hierarchy. > > This results in a lot of bogus lockdep splats when running stuff on > systems with multiple gpus. Partially remedy the situation by only > doing might_lock checks on drivers that do use struct_mutex still for > gem locking. > > A more complete solution would be to do the mutex_init in the drm core > only for legacy drivers, plus add it to each modern driver that still > needs it, which would also give each its own lockdep key. Trying to do > that dynamically doesn't work, because lockdep requires it's keys to > be statically allocated. I placed it in drm_driver which is static to get around that, did you see the patch adding drm_driver_class? I didn't choose that path since this might_lock clearly belongs to kref_put_mutex... Similarly might_lock can also include might_sleep +#define lock_is_mutex(lock) \ + __builtin_types_compatible_p(typeof(*lock), struct mutex) + # define might_lock(lock) \ do { \ + might_sleep_if(lock_is_mutex(lock)); > Cc: Hans de Goede <hdegoede@redhat.com> > Cc: Ben Skeggs <bskeggs@redhat.com> > Cc: Jiri Slaby <jirislaby@gmail.com> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Ingo Molnar <mingo@redhat.com> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com> > --- > drivers/gpu/drm/drm_gem.c | 8 +++++--- > 1 file changed, 5 insertions(+), 3 deletions(-) > > diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c > index 8dc11064253d..9663a79dd363 100644 > --- a/drivers/gpu/drm/drm_gem.c > +++ b/drivers/gpu/drm/drm_gem.c > @@ -826,13 +826,15 @@ drm_gem_object_put_unlocked(struct drm_gem_object *obj) > return; > > dev = obj->dev; > - might_lock(&dev->struct_mutex); > > 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, > + else { > + might_lock(&dev->struct_mutex); > + if (kref_put_mutex(&obj->refcount, drm_gem_object_free, > &dev->struct_mutex)) > - mutex_unlock(&dev->struct_mutex); > + mutex_unlock(&dev->struct_mutex); > + } > } > EXPORT_SYMBOL(drm_gem_object_put_unlocked);
Quoting Daniel Vetter (2017-07-15 10:53:28) > For modern drivers the DRM core doesn't use struct_mutex at all, which > means it's defacto a driver-private lock. But since we still need it > for legacy drivers we can't initialize it in drivers, which means all > the different instances share one lockdep key. Despite that they might > be placed in totally different places in the locking hierarchy. > > This results in a lot of bogus lockdep splats when running stuff on > systems with multiple gpus. Partially remedy the situation by only > doing might_lock checks on drivers that do use struct_mutex still for > gem locking. > > A more complete solution would be to do the mutex_init in the drm core > only for legacy drivers, plus add it to each modern driver that still > needs it, which would also give each its own lockdep key. Trying to do > that dynamically doesn't work, because lockdep requires it's keys to > be statically allocated. > > Cc: Hans de Goede <hdegoede@redhat.com> > Cc: Ben Skeggs <bskeggs@redhat.com> > Cc: Jiri Slaby <jirislaby@gmail.com> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Ingo Molnar <mingo@redhat.com> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com> But fwiw, the patch isn't wrong and fixes a false positive, so Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> > drivers/gpu/drm/drm_gem.c | 8 +++++--- > 1 file changed, 5 insertions(+), 3 deletions(-) > > diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c > index 8dc11064253d..9663a79dd363 100644 > --- a/drivers/gpu/drm/drm_gem.c > +++ b/drivers/gpu/drm/drm_gem.c > @@ -826,13 +826,15 @@ drm_gem_object_put_unlocked(struct drm_gem_object *obj) > return; > > dev = obj->dev; > - might_lock(&dev->struct_mutex); > > if (dev->driver->gem_free_object_unlocked) coding-style nit, if one branch needs {} they all need {}. (The real reason why I didn't want to move might_lock around in this function ;) > kref_put(&obj->refcount, drm_gem_object_free); > - else if (kref_put_mutex(&obj->refcount, drm_gem_object_free, > + else { > + might_lock(&dev->struct_mutex); > + if (kref_put_mutex(&obj->refcount, drm_gem_object_free, > &dev->struct_mutex)) > - mutex_unlock(&dev->struct_mutex); > + mutex_unlock(&dev->struct_mutex); > + } > } > EXPORT_SYMBOL(drm_gem_object_put_unlocked);
On Sat, Jul 15, 2017 at 12:02:47PM +0100, Chris Wilson wrote: > Quoting Daniel Vetter (2017-07-15 10:53:28) > > For modern drivers the DRM core doesn't use struct_mutex at all, which > > means it's defacto a driver-private lock. But since we still need it > > for legacy drivers we can't initialize it in drivers, which means all > > the different instances share one lockdep key. Despite that they might > > be placed in totally different places in the locking hierarchy. > > > > This results in a lot of bogus lockdep splats when running stuff on > > systems with multiple gpus. Partially remedy the situation by only > > doing might_lock checks on drivers that do use struct_mutex still for > > gem locking. > > > > A more complete solution would be to do the mutex_init in the drm core > > only for legacy drivers, plus add it to each modern driver that still > > needs it, which would also give each its own lockdep key. Trying to do > > that dynamically doesn't work, because lockdep requires it's keys to > > be statically allocated. > > > > Cc: Hans de Goede <hdegoede@redhat.com> > > Cc: Ben Skeggs <bskeggs@redhat.com> > > Cc: Jiri Slaby <jirislaby@gmail.com> > > Cc: Peter Zijlstra <peterz@infradead.org> > > Cc: Ingo Molnar <mingo@redhat.com> > > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com> > > But fwiw, the patch isn't wrong and fixes a false positive, so > Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> > > > drivers/gpu/drm/drm_gem.c | 8 +++++--- > > 1 file changed, 5 insertions(+), 3 deletions(-) > > > > diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c > > index 8dc11064253d..9663a79dd363 100644 > > --- a/drivers/gpu/drm/drm_gem.c > > +++ b/drivers/gpu/drm/drm_gem.c > > @@ -826,13 +826,15 @@ drm_gem_object_put_unlocked(struct drm_gem_object *obj) > > return; > > > > dev = obj->dev; > > - might_lock(&dev->struct_mutex); > > > > if (dev->driver->gem_free_object_unlocked) > > coding-style nit, if one branch needs {} they all need {}. > (The real reason why I didn't want to move might_lock around in this > function ;) Fixed while applying, thanks for your review. -Daniel > > > kref_put(&obj->refcount, drm_gem_object_free); > > - else if (kref_put_mutex(&obj->refcount, drm_gem_object_free, > > + else { > > + might_lock(&dev->struct_mutex); > > + if (kref_put_mutex(&obj->refcount, drm_gem_object_free, > > &dev->struct_mutex)) > > - mutex_unlock(&dev->struct_mutex); > > + mutex_unlock(&dev->struct_mutex); > > + } > > } > > EXPORT_SYMBOL(drm_gem_object_put_unlocked); > _______________________________________________ > dri-devel mailing list > dri-devel@lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/dri-devel
diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c index 8dc11064253d..9663a79dd363 100644 --- a/drivers/gpu/drm/drm_gem.c +++ b/drivers/gpu/drm/drm_gem.c @@ -826,13 +826,15 @@ drm_gem_object_put_unlocked(struct drm_gem_object *obj) return; dev = obj->dev; - might_lock(&dev->struct_mutex); 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, + else { + might_lock(&dev->struct_mutex); + if (kref_put_mutex(&obj->refcount, drm_gem_object_free, &dev->struct_mutex)) - mutex_unlock(&dev->struct_mutex); + mutex_unlock(&dev->struct_mutex); + } } EXPORT_SYMBOL(drm_gem_object_put_unlocked);
For modern drivers the DRM core doesn't use struct_mutex at all, which means it's defacto a driver-private lock. But since we still need it for legacy drivers we can't initialize it in drivers, which means all the different instances share one lockdep key. Despite that they might be placed in totally different places in the locking hierarchy. This results in a lot of bogus lockdep splats when running stuff on systems with multiple gpus. Partially remedy the situation by only doing might_lock checks on drivers that do use struct_mutex still for gem locking. A more complete solution would be to do the mutex_init in the drm core only for legacy drivers, plus add it to each modern driver that still needs it, which would also give each its own lockdep key. Trying to do that dynamically doesn't work, because lockdep requires it's keys to be statically allocated. Cc: Hans de Goede <hdegoede@redhat.com> Cc: Ben Skeggs <bskeggs@redhat.com> Cc: Jiri Slaby <jirislaby@gmail.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Ingo Molnar <mingo@redhat.com> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com> --- drivers/gpu/drm/drm_gem.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-)