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Subject[PATCH] drm: Use srcu to protect drm_device.unplugged
Date
From: Noralf Trønnes <noralf@tronnes.org>

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
---
drivers/gpu/drm/drm_drv.c | 54 ++++++++++++++++++++++++++++++++++++++++++-----
include/drm/drm_device.h | 9 +++++++-
include/drm/drm_drv.h | 15 +++++++++----
3 files changed, 68 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index a1b9338736e3..32a83b41ab61 100644
--- a/drivers/gpu/drm/drm_drv.c
+++ b/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)
+{
+ *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)
{
- smp_wmb();
- atomic_set(&dev->unplugged, 1);
+ 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);

diff --git a/include/drm/drm_device.h b/include/drm/drm_device.h
index 7c4fa32f3fc6..3a0eac2885b7 100644
--- a/include/drm/drm_device.h
+++ b/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 */
/*@} */
diff --git a/include/drm/drm_drv.h b/include/drm/drm_drv.h
index d23dcdd1bd95..7e545f5f94d3 100644
--- a/include/drm/drm_drv.h
+++ b/include/drm/drm_drv.h
@@ -624,6 +624,8 @@ 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);

/**
@@ -635,11 +637,16 @@ 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;
}


--
2.7.4
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