drm: Use srcu to protect drm_device.unplugged

Use srcu to protect drm_device.unplugged in a race free manner.
Drivers can use drm_dev_enter()/drm_dev_exit() to protect and mark
sections preventing access to device resources that are not available
after the device is gone.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Tested-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Cc: intel-gfx@lists.freedesktop.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1522222715-11814-1-git-send-email-andr2000@gmail.com

drivers/gpu/drm/drm_drv.c

@@ -32,6 +32,7 @@
 #include <linux/moduleparam.h>
 #include <linux/mount.h>
 #include <linux/slab.h>
+#include <linux/srcu.h>
 
 #include <drm/drm_drv.h>
 #include <drm/drmP.h>
@@ -75,6 +76,8 @@ static bool drm_core_init_complete = false;
 
 static struct dentry *drm_debugfs_root;
 
+DEFINE_STATIC_SRCU(drm_unplug_srcu);
+
 /*
  * DRM Minors
  * A DRM device can provide several char-dev interfaces on the DRM-Major. Each
@@ -318,18 +321,51 @@ void drm_put_dev(struct drm_device *dev)
 }
 EXPORT_SYMBOL(drm_put_dev);
 
-static void drm_device_set_unplugged(struct drm_device *dev)
+/**
+ * drm_dev_enter - Enter device critical section
+ * @dev: DRM device
+ * @idx: Pointer to index that will be passed to the matching drm_dev_exit()
+ *
+ * This function marks and protects the beginning of a section that should not
+ * be entered after the device has been unplugged. The section end is marked
+ * with drm_dev_exit(). Calls to this function can be nested.
+ *
+ * Returns:
+ * True if it is OK to enter the section, false otherwise.
+ */
+bool drm_dev_enter(struct drm_device *dev, int *idx)
 {
-	smp_wmb();
-	atomic_set(&dev->unplugged, 1);
+	*idx = srcu_read_lock(&drm_unplug_srcu);
+
+	if (dev->unplugged) {
+		srcu_read_unlock(&drm_unplug_srcu, *idx);
+		return false;
+	}
+
+	return true;
 }
+EXPORT_SYMBOL(drm_dev_enter);
+
+/**
+ * drm_dev_exit - Exit device critical section
+ * @idx: index returned from drm_dev_enter()
+ *
+ * This function marks the end of a section that should not be entered after
+ * the device has been unplugged.
+ */
+void drm_dev_exit(int idx)
+{
+	srcu_read_unlock(&drm_unplug_srcu, idx);
+}
+EXPORT_SYMBOL(drm_dev_exit);
 
 /**
  * drm_dev_unplug - unplug a DRM device
  * @dev: DRM device
  *
  * This unplugs a hotpluggable DRM device, which makes it inaccessible to
- * userspace operations. Entry-points can use drm_dev_is_unplugged(). This
+ * userspace operations. Entry-points can use drm_dev_enter() and
+ * drm_dev_exit() to protect device resources in a race free manner. This
  * essentially unregisters the device like drm_dev_unregister(), but can be
  * called while there are still open users of @dev.
  */
@@ -338,10 +374,18 @@ void drm_dev_unplug(struct drm_device *dev)
 	drm_dev_unregister(dev);
 
 	mutex_lock(&drm_global_mutex);
-	drm_device_set_unplugged(dev);
 	if (dev->open_count == 0)
 		drm_dev_put(dev);
 	mutex_unlock(&drm_global_mutex);
+
+	/*
+	 * After synchronizing any critical read section is guaranteed to see
+	 * the new value of ->unplugged, and any critical section which might
+	 * still have seen the old value of ->unplugged is guaranteed to have
+	 * finished.
+	 */
+	dev->unplugged = true;
+	synchronize_srcu(&drm_unplug_srcu);
 }
 EXPORT_SYMBOL(drm_dev_unplug);
 

include/drm/drm_device.h

@@ -46,7 +46,14 @@ struct drm_device {
 	/* currently active master for this device. Protected by master_mutex */
 	struct drm_master *master;
 
-	atomic_t unplugged;			/**< Flag whether dev is dead */
+	/**
+	 * @unplugged:
+	 *
+	 * Flag to tell if the device has been unplugged.
+	 * See drm_dev_enter() and drm_dev_is_unplugged().
+	 */
+	bool unplugged;
+
 	struct inode *anon_inode;		/**< inode for private address-space */
 	char *unique;				/**< unique name of the device */
 	/*@} */

include/drm/drm_drv.h

@@ -623,6 +623,8 @@ void drm_dev_get(struct drm_device *dev);
 void drm_dev_put(struct drm_device *dev);
 void drm_dev_unref(struct drm_device *dev);
 void drm_put_dev(struct drm_device *dev);
+bool drm_dev_enter(struct drm_device *dev, int *idx);
+void drm_dev_exit(int idx);
 void drm_dev_unplug(struct drm_device *dev);
 
 /**
@@ -634,11 +636,16 @@ void drm_dev_unplug(struct drm_device *dev);
  * unplugged, these two functions guarantee that any store before calling
  * drm_dev_unplug() is visible to callers of this function after it completes
  */
-static inline int drm_dev_is_unplugged(struct drm_device *dev)
+static inline bool drm_dev_is_unplugged(struct drm_device *dev)
 {
-	int ret = atomic_read(&dev->unplugged);
-	smp_rmb();
-	return ret;
+	int idx;
+
+	if (drm_dev_enter(dev, &idx)) {
+		drm_dev_exit(idx);
+		return false;
+	}
+
+	return true;
 }