[PATCH v2] drm: Release filp before global lock

[Chris Wilson]

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.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Thomas Hellström (VMware) <thomas_os at shipmail.org>
---
 drivers/gpu/drm/drm_file.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
index 92d16724f949..777b450870a5 100644
--- a/drivers/gpu/drm/drm_file.c
+++ b/drivers/gpu/drm/drm_file.c
@@ -220,7 +220,7 @@ void drm_file_free(struct drm_file *file)
 	DRM_DEBUG("pid = %d, device = 0x%lx, open_count = %d\n",
 		  task_pid_nr(current),
 		  (long)old_encode_dev(file->minor->kdev->devt),
-		  dev->open_count);
+		  READ_ONCE(dev->open_count));
 
 	if (drm_core_check_feature(dev, DRIVER_LEGACY) &&
 	    dev->driver->preclose)
@@ -438,12 +438,10 @@ int drm_release(struct inode *inode, struct file *filp)
 	struct drm_minor *minor = file_priv->minor;
 	struct drm_device *dev = minor->dev;
 
-	mutex_lock(&drm_global_mutex);
-
-	DRM_DEBUG("open_count = %d\n", dev->open_count);
-
 	drm_close_helper(filp);
 
+	mutex_lock(&drm_global_mutex);
+
 	if (!--dev->open_count)
 		drm_lastclose(dev);
 
-- 
2.25.0