drm: kill BKL from common code

This restricts the use of the big kernel lock to the i830 and i810
device drivers. The three remaining users in common code (open, ioctl
and release) get converted to a new mutex, the drm_global_mutex,
making the locking stricter than the big kernel lock.

This may have a performance impact, but only in those cases that
currently don't use DRM_UNLOCKED flag in the ioctl list and would
benefit from that anyway.

The reason why i810 and i830 cannot use drm_global_mutex in their
mmap functions is a lock-order inversion problem between the current
use of the BKL and mmap_sem in these drivers. Since the BKL has
release-on-sleep semantics, it's harmless but it would cause trouble
if we replace the BKL with a mutex.

Instead, these drivers get their own ioctl wrappers that take the
BKL around every ioctl call and then set their own handlers as
DRM_UNLOCKED.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 11 insertions, 12 deletions

diff --git a/drivers/gpu/drm/drm_fops.c b/drivers/gpu/drm/drm_fops.c
index e7aace20981f..2ca8df8b6102 100644
--- a/drivers/gpu/drm/drm_fops.c
+++ b/drivers/gpu/drm/drm_fops.c
@@ -39,6 +39,9 @@
 #include <linux/slab.h>
 #include <linux/smp_lock.h>
 
+/* from BKL pushdown: note that nothing else serializes idr_find() */
+DEFINE_MUTEX(drm_global_mutex);
+
 static int drm_open_helper(struct inode *inode, struct file *filp,
 			   struct drm_device * dev);
 
@@ -175,8 +178,7 @@ int drm_stub_open(struct inode *inode, struct file *filp)
 
 	DRM_DEBUG("\n");
 
-	/* BKL pushdown: note that nothing else serializes idr_find() */
-	lock_kernel();
+	mutex_lock(&drm_global_mutex);
 	minor = idr_find(&drm_minors_idr, minor_id);
 	if (!minor)
 		goto out;
@@ -197,7 +199,7 @@ int drm_stub_open(struct inode *inode, struct file *filp)
 	fops_put(old_fops);
 
 out:
-	unlock_kernel();
+	mutex_unlock(&drm_global_mutex);
 	return err;
 }
 
@@ -472,7 +474,7 @@ int drm_release(struct inode *inode, struct file *filp)
 	struct drm_device *dev = file_priv->minor->dev;
 	int retcode = 0;
 
-	lock_kernel();
+	mutex_lock(&drm_global_mutex);
 
 	DRM_DEBUG("open_count = %d\n", dev->open_count);
 
@@ -573,17 +575,14 @@ int drm_release(struct inode *inode, struct file *filp)
 		if (atomic_read(&dev->ioctl_count)) {
 			DRM_ERROR("Device busy: %d\n",
 				  atomic_read(&dev->ioctl_count));
-			spin_unlock(&dev->count_lock);
-			unlock_kernel();
-			return -EBUSY;
+			retcode = -EBUSY;
+			goto out;
 		}
-		spin_unlock(&dev->count_lock);
-		unlock_kernel();
-		return drm_lastclose(dev);
+		retcode = drm_lastclose(dev);
 	}
+out:
 	spin_unlock(&dev->count_lock);
-
-	unlock_kernel();
+	mutex_unlock(&drm_global_mutex);
 
 	return retcode;
 }