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authorDesmond Cheong Zhi Xi2021-07-12 12:35:07 +0800
committerDaniel Vetter2021-07-20 20:17:58 +0200
commit0b0860a3cf5eccf183760b1177a1dcdb821b0b66 (patch)
treed8893b9878ac83636e71fc5113443a8219d4968a /include/drm
parent1f7ef07cfa14fb8557d1f1b7a14c76926142a4fb (diff)
drm: serialize drm_file.master with a new spinlock
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 spin lock 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_lookup_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_lookup_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_lookup_lock. Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch> Link: https://patchwork.freedesktop.org/patch/msgid/20210712043508.11584-5-desmondcheongzx@gmail.com
Diffstat (limited to 'include/drm')
-rw-r--r--include/drm/drm_file.h12
1 files changed, 9 insertions, 3 deletions
diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
index b81b3bfb08c8..9b82988e3427 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_lookup_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. */
+ spinlock_t master_lookup_lock;
+
/** @pid: Process that opened this file. */
struct pid *pid;