drm/i915: Avoid might_fault during pwrite whilst holding our mutex

... and so prevent a potential circular reference:

  [ INFO: possible circular locking dependency detected ]
  2.6.37-rc1-uwe1+ #4
  -------------------------------------------------------
  Xorg/1401 is trying to acquire lock:
   (&mm->mmap_sem){++++++}, at: [<c01e4ddb>] might_fault+0x4b/0xa0

  but task is already holding lock:
   (&dev->struct_mutex){+.+.+.}, at: [<f869c3ac>]
  i915_mutex_lock_interruptible+0x3c/0x60 [i915]

  which lock already depends on the new lock.

When the locking around the pwrite ioctl was simplified, I did not spot
that the phys path never took any locks and so we introduced this
potential circular reference.
Reported-by: Uwe Helm <uwe.helm@googlemail.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_gem.c

@@ -4878,17 +4878,24 @@ i915_gem_phys_pwrite(struct drm_device *dev, struct drm_gem_object *obj,
 		     struct drm_file *file_priv)
 {
 	struct drm_i915_gem_object *obj_priv = to_intel_bo(obj);
-	void *obj_addr;
-	int ret;
-	char __user *user_data;
+	void *vaddr = obj_priv->phys_obj->handle->vaddr + args->offset;
+	char __user *user_data = (char __user *) (uintptr_t) args->data_ptr;
 
-	user_data = (char __user *) (uintptr_t) args->data_ptr;
-	obj_addr = obj_priv->phys_obj->handle->vaddr + args->offset;
+	DRM_DEBUG_DRIVER("vaddr %p, %lld\n", vaddr, args->size);
 
-	DRM_DEBUG_DRIVER("obj_addr %p, %lld\n", obj_addr, args->size);
-	ret = copy_from_user(obj_addr, user_data, args->size);
-	if (ret)
+	if (__copy_from_user_inatomic_nocache(vaddr, user_data, args->size)) {
+		unsigned long unwritten;
+
+		/* The physical object once assigned is fixed for the lifetime
+		 * of the obj, so we can safely drop the lock and continue
+		 * to access vaddr.
+		 */
+		mutex_unlock(&dev->struct_mutex);
+		unwritten = copy_from_user(vaddr, user_data, args->size);
+		mutex_lock(&dev->struct_mutex);
+		if (unwritten)
 			return -EFAULT;
+	}
 
 	drm_agp_chipset_flush(dev);
 	return 0;