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drm: provide device-refcount

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Lets not trick ourselves into thinking "drm_device" objects are not
ref-counted. That's just utterly stupid. We manage "drm_minor" objects on
each drm-device and each minor can have an unlimited number of open
handles. Each of these handles has the drm_minor (and thus the drm_device)
as private-data in the file-handle. Therefore, we may not destroy
"drm_device" until all these handles are closed.

It is *not* possible to reset all these pointers atomically and restrict
access to them, and this is *not* how this is done! Instead, we use
ref-counts to make sure the object is valid and not freed.

Note that we currently use "dev->open_count" for that, which is *exactly*
the same as a reference-count, just open coded. So this patch doesn't
change any semantics on DRM devices (well, this patch just introduces the
ref-count, anyway. Follow-up patches will replace open_count by it).

Also note that generic VFS revoke support could allow us to drop this
ref-count again. We could then just synchronously disable any fops->xy()
calls. However, this is not the case, yet, and no such patches are
in sight (and I seriously question the idea of dropping the ref-cnt
again).

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
This commit is contained in:
David Herrmann
2014-01-29 10:21:36 +01:00
parent cb8a239b03
commit 099d1c290e
6 changed files with 50 additions and 19 deletions
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5
include/drm/drmP.h
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@@ -43,6 +43,7 @@
#include <asm/current.h>
#endif /* __alpha__ */
#include <linux/kernel.h>
#include <linux/kref.h>
#include <linux/miscdevice.h>
#include <linux/fs.h>
#include <linux/init.h>
@@ -1102,6 +1103,7 @@ struct drm_device {
/** \name Lifetime Management */
/*@{ */
struct kref ref; /**< Object ref-count */
struct device *dev; /**< Device structure of bus-device */
struct drm_driver *driver; /**< DRM driver managing the device */
void *dev_private; /**< DRM driver private data */
@@ -1666,7 +1668,8 @@ static __inline__ void drm_core_dropmap(struct drm_local_map *map)
struct drm_device *drm_dev_alloc(struct drm_driver *driver,
struct device *parent);
void drm_dev_free(struct drm_device *dev);
void drm_dev_ref(struct drm_device *dev);
void drm_dev_unref(struct drm_device *dev);
int drm_dev_register(struct drm_device *dev, unsigned long flags);
void drm_dev_unregister(struct drm_device *dev);
/*@}*/