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Subject[065/103] drm: Hold the mutex when dropping the last GEM reference (v2)
2.6.35-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Chris Wilson <chris@chris-wilson.co.uk>

commit 39b4d07aa3583ceefe73622841303a0a3e942ca1 upstream.

In order to be fully threadsafe we need to check that the drm_gem_object
refcount is still 0 after acquiring the mutex in order to call the free
function. Otherwise, we may encounter scenarios like:

Thread A: Thread B:
drm_gem_close
unreference_unlocked
kref_put mutex_lock
... i915_gem_evict
... kref_get -> BUG
... i915_gem_unbind
... kref_put
... i915_gem_object_free
... mutex_unlock
mutex_lock
i915_gem_object_free -> BUG
i915_gem_object_unbind
kfree
mutex_unlock

Note that no driver is currently using the free_unlocked vfunc and it is
scheduled for removal, hasten that process.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=30454
Reported-and-Tested-by: Magnus Kessler <Magnus.Kessler@gmx.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
drivers/gpu/drm/drm_gem.c | 22 ----------------------
include/drm/drmP.h | 10 ++++++----
2 files changed, 6 insertions(+), 26 deletions(-)

--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -451,28 +451,6 @@ drm_gem_object_free(struct kref *kref)
}
EXPORT_SYMBOL(drm_gem_object_free);

-/**
- * Called after the last reference to the object has been lost.
- * Must be called without holding struct_mutex
- *
- * Frees the object
- */
-void
-drm_gem_object_free_unlocked(struct kref *kref)
-{
- struct drm_gem_object *obj = (struct drm_gem_object *) kref;
- struct drm_device *dev = obj->dev;
-
- if (dev->driver->gem_free_object_unlocked != NULL)
- dev->driver->gem_free_object_unlocked(obj);
- else if (dev->driver->gem_free_object != NULL) {
- mutex_lock(&dev->struct_mutex);
- dev->driver->gem_free_object(obj);
- mutex_unlock(&dev->struct_mutex);
- }
-}
-EXPORT_SYMBOL(drm_gem_object_free_unlocked);
-
static void drm_gem_object_ref_bug(struct kref *list_kref)
{
BUG();
--- a/include/drm/drmP.h
+++ b/include/drm/drmP.h
@@ -802,7 +802,6 @@ struct drm_driver {
*/
int (*gem_init_object) (struct drm_gem_object *obj);
void (*gem_free_object) (struct drm_gem_object *obj);
- void (*gem_free_object_unlocked) (struct drm_gem_object *obj);

/* vga arb irq handler */
void (*vgaarb_irq)(struct drm_device *dev, bool state);
@@ -1431,7 +1430,6 @@ int drm_gem_init(struct drm_device *dev)
void drm_gem_destroy(struct drm_device *dev);
void drm_gem_object_release(struct drm_gem_object *obj);
void drm_gem_object_free(struct kref *kref);
-void drm_gem_object_free_unlocked(struct kref *kref);
struct drm_gem_object *drm_gem_object_alloc(struct drm_device *dev,
size_t size);
int drm_gem_object_init(struct drm_device *dev,
@@ -1457,8 +1455,12 @@ drm_gem_object_unreference(struct drm_ge
static inline void
drm_gem_object_unreference_unlocked(struct drm_gem_object *obj)
{
- if (obj != NULL)
- kref_put(&obj->refcount, drm_gem_object_free_unlocked);
+ if (obj != NULL) {
+ struct drm_device *dev = obj->dev;
+ mutex_lock(&dev->struct_mutex);
+ kref_put(&obj->refcount, drm_gem_object_free);
+ mutex_unlock(&dev->struct_mutex);
+ }
}

int drm_gem_handle_create(struct drm_file *file_priv,



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