drm: Nerf drm_global_mutex BKL for good drivers

This catches the majority of drivers (unfortunately not if we take
users into account, because all the big drivers have at least a
lastclose hook).

With the prep patches out of the way all drm state is fully protected
and either prevents or can deal with the races from dropping the BKL
around open/close. The only thing left to audit are the various driver
hooks - by keeping the BKL around if any of them are set we have a
very simple cop-out!

Note that one of the biggest prep pieces to get here was making
dev->open_count atomic, which was done in

commit 7e13ad896484a0165a68197a2e64091ea28c9602
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Jan 24 13:01:07 2020 +0000

    drm: Avoid drm_global_mutex for simple inc/dec of dev->open_count

v2:
- Rebase and fix locking in drm_open() (Chris)
- Indentation fix in drm_release
- Typo fix in the commit message (Sam)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200204150146.2006481-6-daniel.vetter@ffwll.ch

1 files changed, 42 insertions, 6 deletions

diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
index 80d556402ab4..c4c704e01961 100644
--- a/drivers/gpu/drm/drm_file.c
+++ b/drivers/gpu/drm/drm_file.c
@@ -51,6 +51,37 @@
 /* from BKL pushdown */
 DEFINE_MUTEX(drm_global_mutex);
 
+bool drm_dev_needs_global_mutex(struct drm_device *dev)
+{
+	/*
+	 * Legacy drivers rely on all kinds of BKL locking semantics, don't
+	 * bother. They also still need BKL locking for their ioctls, so better
+	 * safe than sorry.
+	 */
+	if (drm_core_check_feature(dev, DRIVER_LEGACY))
+		return true;
+
+	/*
+	 * The deprecated ->load callback must be called after the driver is
+	 * already registered. This means such drivers rely on the BKL to make
+	 * sure an open can't proceed until the driver is actually fully set up.
+	 * Similar hilarity holds for the unload callback.
+	 */
+	if (dev->driver->load || dev->driver->unload)
+		return true;
+
+	/*
+	 * Drivers with the lastclose callback assume that it's synchronized
+	 * against concurrent opens, which again needs the BKL. The proper fix
+	 * is to use the drm_client infrastructure with proper locking for each
+	 * client.
+	 */
+	if (dev->driver->lastclose)
+		return true;
+
+	return false;
+}
+
 /**
  * DOC: file operations
  *
@@ -378,9 +409,10 @@ int drm_open(struct inode *inode, struct file *filp)
 	if (IS_ERR(minor))
 		return PTR_ERR(minor);
 
-	mutex_lock(&drm_global_mutex);
-
 	dev = minor->dev;
+	if (drm_dev_needs_global_mutex(dev))
+		mutex_lock(&drm_global_mutex);
+
 	if (!atomic_fetch_inc(&dev->open_count))
 		need_setup = 1;
 
@@ -398,13 +430,15 @@ int drm_open(struct inode *inode, struct file *filp)
 		}
 	}
 
-	mutex_unlock(&drm_global_mutex);
+	if (drm_dev_needs_global_mutex(dev))
+		mutex_unlock(&drm_global_mutex);
 
 	return 0;
 
 err_undo:
 	atomic_dec(&dev->open_count);
-	mutex_unlock(&drm_global_mutex);
+	if (drm_dev_needs_global_mutex(dev))
+		mutex_unlock(&drm_global_mutex);
 	drm_minor_release(minor);
 	return retcode;
 }
@@ -444,7 +478,8 @@ 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);
+	if (drm_dev_needs_global_mutex(dev))
+		mutex_lock(&drm_global_mutex);
 
 	DRM_DEBUG("open_count = %d\n", atomic_read(&dev->open_count));
 
@@ -453,7 +488,8 @@ int drm_release(struct inode *inode, struct file *filp)
 	if (atomic_dec_and_test(&dev->open_count))
 		drm_lastclose(dev);
 
-	mutex_unlock(&drm_global_mutex);
+	if (drm_dev_needs_global_mutex(dev))
+		mutex_unlock(&drm_global_mutex);
 
 	drm_minor_release(minor);