mm: change mm_update_next_owner() to update mm->owner with WRITE_ONCE

The RCU reader uses rcu_dereference() inside rcu_read_lock critical
sections, so the writer shall use WRITE_ONCE.  Just a cleanup, we still
rely on gcc to emit atomic writes in other places.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Andrea Arcangeli <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: "Kirill A . Shutemov" <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: zhong jiang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

kernel/exit.c

@@ -422,7 +422,7 @@ void mm_update_next_owner(struct mm_struct *mm)
 	 * freed task structure.
 	 */
 	if (atomic_read(&mm->mm_users) <= 1) {
-		mm->owner = NULL;
+		WRITE_ONCE(mm->owner, NULL);
 		return;
 	}
 
@@ -462,7 +462,7 @@ void mm_update_next_owner(struct mm_struct *mm)
 	 * most likely racing with swapoff (try_to_unuse()) or /proc or
 	 * ptrace or page migration (get_task_mm()).  Mark owner as NULL.
 	 */
-	mm->owner = NULL;
+	WRITE_ONCE(mm->owner, NULL);
 	return;
 
 assign_new_owner:
@@ -483,7 +483,7 @@ void mm_update_next_owner(struct mm_struct *mm)
 		put_task_struct(c);
 		goto retry;
 	}
-	mm->owner = c;
+	WRITE_ONCE(mm->owner, c);
 	task_unlock(c);
 	put_task_struct(c);
 }