1 files changed, 14 insertions, 7 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 40036ac04271..059b38cae384 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -580,8 +580,6 @@ mm_need_new_owner(struct mm_struct *mm, struct task_struct *p)
          * If there are other users of the mm and the owner (us) is exiting
          * we need to find a new owner to take on the responsibility.
          */
-        if (!mm)
-                return 0;
         if (atomic_read(&mm->mm_users) <= 1)
                 return 0;
         if (mm->owner != p)
@@ -624,29 +622,38 @@ retry:
         } while_each_thread(g, c);
 
         read_unlock(&tasklist_lock);
+        /*
+         * We found no owner yet mm_users > 1: this implies that we are
+         * most likely racing with swapoff (try_to_unuse()) or /proc or
+         * ptrace or page migration (get_task_mm()).  Mark owner as NULL,
+         * so that subsystems can understand the callback and take action.
+         */
+        down_write(&mm->mmap_sem);
+        cgroup_mm_owner_callbacks(mm->owner, NULL);
+        mm->owner = NULL;
+        up_write(&mm->mmap_sem);
         return;
 
 assign_new_owner:
         BUG_ON(c == p);
         get_task_struct(c);
+        read_unlock(&tasklist_lock);
+        down_write(&mm->mmap_sem);
         /*
          * The task_lock protects c->mm from changing.
          * We always want mm->owner->mm == mm
          */
         task_lock(c);
-        /*
-         * Delay read_unlock() till we have the task_lock()
-         * to ensure that c does not slip away underneath us
-         */
-        read_unlock(&tasklist_lock);
         if (c->mm != mm) {
                 task_unlock(c);
+                up_write(&mm->mmap_sem);
                 put_task_struct(c);
                 goto retry;
         }
         cgroup_mm_owner_callbacks(mm->owner, c);
         mm->owner = c;
         task_unlock(c);
+        up_write(&mm->mmap_sem);
         put_task_struct(c);
 }
 #endif /* CONFIG_MM_OWNER */