drm/i915: Fixup userptr mmu notifier registration error handling

Avoid dereferencing the error pointer and also avoid returning NULL
from i915_mmu_notifier_find since the callers do not expect that.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 7741b547b6e0 ("drm/i915: Preallocate our mmu notifier workequeu to unbreak cpu hotplug deadlock")
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171017150908.12840-1-tvrtko.ursulin@linux.intel.com

diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
index cdc9be7..e26b231 100644 (file)
--- a/drivers/gpu/drm/i915/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
@@ -221,15 +221,17 @@ i915_mmu_notifier_find(struct i915_mm_struct *mm)
                        /* Protected by mm_lock */
                        mm->mn = fetch_and_zero(&mn);
                }
-       } else {
-               /* someone else raced and successfully installed the mmu
-                * notifier, we can cancel our own errors */
+       } else if (mm->mn) {
+               /*
+                * Someone else raced and successfully installed the mmu
+                * notifier, we can cancel our own errors.
+                */
                err = 0;
        }
        mutex_unlock(&mm->i915->mm_lock);
        up_write(&mm->mm->mmap_sem);
 
-       if (mn) {
+       if (mn && !IS_ERR(mn)) {
                destroy_workqueue(mn->wq);
                kfree(mn);
        }