Currently, drm_file.master pointers should be protected by drm_device.master_mutex when being dereferenced. This is because drm_file.master is not invariant for the lifetime of drm_file. If drm_file is not the creator of master, then drm_file.is_master is false, and a call to drm_setmaster_ioctl will invoke drm_new_set_master, which then allocates a new master for drm_file and puts the old master.
Thus, without holding drm_device.master_mutex, the old value of drm_file.master could be freed while it is being used by another concurrent process.
However, it is not always possible to lock drm_device.master_mutex to dereference drm_file.master. Through the fbdev emulation code, this might occur in a deep nest of other locks. But drm_device.master_mutex is also the outermost lock in the nesting hierarchy, so this leads to potential deadlocks.
To address this, we introduce a new mutex at the bottom of the lock hierarchy that only serializes drm_file.master. With this change, the value of drm_file.master changes only when both drm_device.master_mutex and drm_file.master_lock are held. Hence, any process holding either of those locks can ensure that the value of drm_file.master will not change concurrently.
Since no lock depends on the new drm_file.master_lock, when drm_file.master is dereferenced, but drm_device.master_mutex cannot be held, we can safely protect the master pointer with drm_file.master_lock.
Reported-by: Daniel Vetter daniel.vetter@ffwll.ch Signed-off-by: Desmond Cheong Zhi Xi desmondcheongzx@gmail.com ---
Since our lock inversions were a result of dev->master_mutex being used to serialize many other things, perhaps a finer grained lock will solve the lockdep issues.
drivers/gpu/drm/drm_auth.c | 10 ++++++++-- drivers/gpu/drm/drm_file.c | 1 + include/drm/drm_file.h | 12 +++++++++--- 3 files changed, 18 insertions(+), 5 deletions(-)
diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c index ab1863c5a5a0..fe5b6adc6133 100644 --- a/drivers/gpu/drm/drm_auth.c +++ b/drivers/gpu/drm/drm_auth.c @@ -169,11 +169,14 @@ static int drm_new_set_master(struct drm_device *dev, struct drm_file *fpriv)
WARN_ON(fpriv->is_master); old_master = fpriv->master; + mutex_lock(&fpriv->master_lock); fpriv->master = drm_master_create(dev); if (!fpriv->master) { fpriv->master = old_master; + mutex_unlock(&fpriv->master_lock); return -ENOMEM; } + mutex_unlock(&fpriv->master_lock);
fpriv->is_master = 1; fpriv->authenticated = 1; @@ -332,10 +335,13 @@ int drm_master_open(struct drm_file *file_priv) * any master object for render clients */ mutex_lock(&dev->master_mutex); - if (!dev->master) + if (!dev->master) { ret = drm_new_set_master(dev, file_priv); - else + } else { + mutex_lock(&file_priv->master_lock); file_priv->master = drm_master_get(dev->master); + mutex_unlock(&file_priv->master_lock); + } mutex_unlock(&dev->master_mutex);
return ret; diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c index d4f0bac6f8f8..8ccadfa1c752 100644 --- a/drivers/gpu/drm/drm_file.c +++ b/drivers/gpu/drm/drm_file.c @@ -176,6 +176,7 @@ struct drm_file *drm_file_alloc(struct drm_minor *minor) init_waitqueue_head(&file->event_wait); file->event_space = 4096; /* set aside 4k for event buffer */
+ mutex_init(&file->master_lock); mutex_init(&file->event_read_lock);
if (drm_core_check_feature(dev, DRIVER_GEM)) diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h index b81b3bfb08c8..88539f93fc8e 100644 --- a/include/drm/drm_file.h +++ b/include/drm/drm_file.h @@ -226,15 +226,21 @@ struct drm_file { /** * @master: * - * Master this node is currently associated with. Only relevant if - * drm_is_primary_client() returns true. Note that this only - * matches &drm_device.master if the master is the currently active one. + * Master this node is currently associated with. Protected by struct + * &drm_device.master_mutex, and serialized by @master_lock. + * + * Only relevant if drm_is_primary_client() returns true. Note that + * this only matches &drm_device.master if the master is the currently + * active one. * * See also @authentication and @is_master and the :ref:`section on * primary nodes and authentication <drm_primary_node>`. */ struct drm_master *master;
+ /** @master_lock: Serializes @master. */ + struct mutex master_lock; + /** @pid: Process that opened this file. */ struct pid *pid;