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drm: Fix drm_release() and device unplug
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If userspace has open fd(s) when drm_dev_unplug() is run, it will result
in drm_dev_unregister() being called twice. First in drm_dev_unplug() and
then later in drm_release() through the call to drm_put_dev().

Since userspace already holds a ref on drm_device through the drm_minor,
it's not necessary to add extra ref counting based on no open file
handles. Instead just drm_dev_put() unconditionally in drm_dev_unplug().

We now have this:
- Userpace holds a ref on drm_device as long as there's open fd(s)
- The driver holds a ref on drm_device as long as it's bound to the
  struct device

When both sides are done with drm_device, it is released.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sean Paul <sean@poorly.run>
Link: https://patchwork.freedesktop.org/patch/msgid/20190208140103.28919-2-noralf@tronnes.org
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Noralf Trønnes committed Feb 21, 2019
1 parent 1fdafbd commit 1ee57d4
Showing 2 changed files with 3 additions and 9 deletions.
6 changes: 1 addition & 5 deletions drivers/gpu/drm/drm_drv.c
Original file line number Diff line number Diff line change
@@ -376,11 +376,7 @@ void drm_dev_unplug(struct drm_device *dev)
synchronize_srcu(&drm_unplug_srcu);

drm_dev_unregister(dev);

mutex_lock(&drm_global_mutex);
if (dev->open_count == 0)
drm_dev_put(dev);
mutex_unlock(&drm_global_mutex);
drm_dev_put(dev);
}
EXPORT_SYMBOL(drm_dev_unplug);

6 changes: 2 additions & 4 deletions drivers/gpu/drm/drm_file.c
Original file line number Diff line number Diff line change
@@ -479,11 +479,9 @@ int drm_release(struct inode *inode, struct file *filp)

drm_file_free(file_priv);

if (!--dev->open_count) {
if (!--dev->open_count)
drm_lastclose(dev);
if (drm_dev_is_unplugged(dev))
drm_put_dev(dev);
}

mutex_unlock(&drm_global_mutex);

drm_minor_release(minor);

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