drm: Release filp before global lock

The file is not part of the global drm resource and can be released
prior to take the global mutex to drop the open_count (and potentially
close) the drm device. As the global mutex is indeed global, not only
within the device but across devices, a slow file release mechanism can
bottleneck the entire system.

However, inside drm_close_helper() there are a number of dev->driver
callbacks that take the drm_device as the first parameter... Worryingly
some of those callbacks may be (implicitly) depending on the global
mutex.

v2: Drop the debug message for the open-count, it's included with the
drm_file_free() debug message -- and for good measure make that up as
reading outside of the mutex.

v3: Separate the calling of the filp cleanup outside of
drm_global_mutex into a new drm_release_noglobal() hook, so that we can
phase the transition. drm/savage relies on the global mutex, and there
may be more, so be cautious.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Thomas Hellström (VMware) <thomas_os@shipmail.org>
Reviewed-by: Thomas Hellström (VMware) <thomas_os@shipmail.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200124125627.125042-1-chris@chris-wilson.co.uk

1 files changed, 1 insertions, 0 deletions

diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
index 8b099b347817..19df8028a6c4 100644
--- a/include/drm/drm_file.h
+++ b/include/drm/drm_file.h
@@ -374,6 +374,7 @@ int drm_open(struct inode *inode, struct file *filp);
 ssize_t drm_read(struct file *filp, char __user *buffer,
 		 size_t count, loff_t *offset);
 int drm_release(struct inode *inode, struct file *filp);
+int drm_release_noglobal(struct inode *inode, struct file *filp);
 __poll_t drm_poll(struct file *filp, struct poll_table_struct *wait);
 int drm_event_reserve_init_locked(struct drm_device *dev,
 				  struct drm_file *file_priv,