The gem flink name holds a reference onto the object itself, and this
self-reference would prevent an flink'ed object from every being
freed. To break that loop we remove the flink name when the last
userspace handle disappears, i.e. when obj->handle_count reaches 0.
Now in gem_open we drop the dev->object_name_lock between the flink
name lookup and actually adding the handle. This means a concurrent
gem_close of the last handle could result in the flink name getting
reaped right inbetween, i.e.
Thread 1 Thread 2
gem_open gem_close
flink -> obj lookup
handle_count drops to 0
remove flink name
create_handle
handle_count++
If someone now flinks this object again, we'll get a new flink name.
We can close this race by removing the lock dropping and making the
entire lookup+handle_create sequence atomic. Unfortunately to still be
able to share the handle_create logic this requires a
handle_create_tail function which drops the lock - we can't hold the
object_name_lock while calling into a driver's ->gem_open callback.
Note that for flink fixing this race isn't really important, since
racing gem_open against gem_close is clearly a userspace bug. And no
matter how the race ends, we won't leak any references.
But with dma-buf where the userspace dma-buf fd itself is refcounted
this is a valid sequence and hence we should fix it. Therefore this
patch here is just a warm-up exercise (and for consistency between
flink buffer sharing and dma-buf buffer sharing with self-imports).
Also note that this extension of the critical section in gem_open
protected by dev->object_name_lock only works because it's now a
mutex: A spinlock would conflict with the potential memory allocation
in idr_preload().
This is exercises by igt/gem_flink_race/flink_name.
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
---
drivers/gpu/drm/drm_gem.c | 42 +++++++++++++++++++++++++++++++-----------
include/drm/drmP.h | 3 +++
2 files changed, 34 insertions(+), 11 deletions(-)
diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index c00e273..0e9a9c7 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -308,23 +308,26 @@ int drm_gem_dumb_destroy(struct drm_file *file,
EXPORT_SYMBOL(drm_gem_dumb_destroy);
/**
- * Create a handle for this object. This adds a handle reference
- * to the object, which includes a regular reference count. Callers
- * will likely want to dereference the object afterwards.
+ * drm_gem_handle_create_tail - internal functions to create a handle
+ *
+ * This expects the dev->object_name_lock to be held already and will drop it
+ * before returning. Used to avoid races in establishing new handles when
+ * importing an object from either an flink name or a dma-buf.
*/
int
-drm_gem_handle_create(struct drm_file *file_priv,
- struct drm_gem_object *obj,
- u32 *handlep)
+drm_gem_handle_create_tail(struct drm_file *file_priv,
+ struct drm_gem_object *obj,
+ u32 *handlep)
{
struct drm_device *dev = obj->dev;
int ret;
+ WARN_ON(!mutex_is_locked(&dev->object_name_lock));
+
/*
* Get the user-visible handle using idr. Preload and perform
* allocation under our spinlock.
*/
- mutex_lock(&dev->object_name_lock);
idr_preload(GFP_KERNEL);
spin_lock(&file_priv->table_lock);
@@ -351,6 +354,21 @@ drm_gem_handle_create(struct drm_file *file_priv,
return 0;
}
+
+/**
+ * Create a handle for this object. This adds a handle reference
+ * to the object, which includes a regular reference count. Callers
+ * will likely want to dereference the object afterwards.
+ */
+int
+drm_gem_handle_create(struct drm_file *file_priv,
+ struct drm_gem_object *obj,
+ u32 *handlep)
+{
+ mutex_lock(&obj->dev->object_name_lock);
+
+ return drm_gem_handle_create_tail(file_priv, obj, handlep);
+}
EXPORT_SYMBOL(drm_gem_handle_create);
@@ -504,13 +522,15 @@ drm_gem_open_ioctl(struct drm_device *dev, void *data,
mutex_lock(&dev->object_name_lock);
obj = idr_find(&dev->object_name_idr, (int) args->name);
- if (obj)
+ if (obj) {
drm_gem_object_reference(obj);
- mutex_unlock(&dev->object_name_lock);
- if (!obj)
+ } else {
+ mutex_unlock(&dev->object_name_lock);
return -ENOENT;
+ }
- ret = drm_gem_handle_create(file_priv, obj, &handle);
+ /* drm_gem_handle_create_tail unlocks dev->object_name_lock. */
+ ret = drm_gem_handle_create_tail(file_priv, obj, &handle);
drm_gem_object_unreference_unlocked(obj);
if (ret)
return ret;
diff --git a/include/drm/drmP.h b/include/drm/drmP.h
index 49e514b..3b346bc 100644
--- a/include/drm/drmP.h
+++ b/include/drm/drmP.h
@@ -1614,6 +1614,9 @@ drm_gem_object_unreference_unlocked(struct drm_gem_object *obj)
}
}
+int drm_gem_handle_create_tail(struct drm_file *file_priv,
+ struct drm_gem_object *obj,
+ u32 *handlep);
int drm_gem_handle_create(struct drm_file *file_priv,
struct drm_gem_object *obj,
u32 *handlep);
--
1.8.3.2
_______________________________________________
dri-devel mailing list
dri-devel@xxxxxxxxxxxxxxxxxxxxx
http://lists.freedesktop.org/mailman/listinfo/dri-devel
