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@xxxxxxxxxxxxxxxxxx>
Cc: Thomas Hellström (VMware) <thomas_os@xxxxxxxxxxxx>
---
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
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