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>

1 files changed, 16 insertions, 9 deletions

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 984eb6e9db03..eba9b1615228 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/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)
-                return -EFAULT;
+        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;