[PATCH] futex: robust futex maybe never be awaked, on rare situation.

[Yunfeng Cui]

I use model checker find a issue of robust and pi futex. On below
situation, the owner can't find something in pi_state_list, while
the requester will be blocked, never be awaked.

CPU0                       CPU1
                           futex_lock_pi
                           /*some cs code*/
futex_lock_pi
  futex_lock_pi_atomic
    ...
    newval = uval | FUTEX_WAITERS;
    ret = lock_pi_update_atomic(uaddr, uval, newval);
    ...
    attach_to_pi_owner
     ....
     p = find_get_task_by_vpid(pid);
     if (!p)
       return handle_exit_race(uaddr, uval, NULL);
       ....
       raw_spin_lock_irq(&p->pi_lock);
       ....
       pi_state = alloc_pi_state();
       ....
                           do_exit->mm_release
                           if (unlikely(tsk->robust_list)) {
                             exit_robust_list(tsk);
                             tsk->robust_list = NULL;
                           }
                           if (unlikely(!list_empty(&tsk->pi_state_list)))
                             exit_pi_state_list(tsk); /*WILL MISS*/
      list_add(&pi_state->list, &p->pi_state_list);
    WILL BLOCKED, NEVER WAKEUP!

Signed-off-by: Yunfeng Cui <cui.yunfeng@xxxxxxxxxx>
Reviewed-by: Bo Wang <wang.bo116@xxxxxxxxxx>
Reviewed-by: Yi Wang <wang.yi59@xxxxxxxxxx>
---
 kernel/fork.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 53e780748fe3..58b90f21dac4 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1277,15 +1277,16 @@ void mm_release(struct task_struct *tsk, struct mm_struct *mm)
 	if (unlikely(tsk->robust_list)) {
 		exit_robust_list(tsk);
 		tsk->robust_list = NULL;
+		/*Check pi_state_list of task on pi_lock be acquired*/
+		exit_pi_state_list(tsk);
 	}
 #ifdef CONFIG_COMPAT
 	if (unlikely(tsk->compat_robust_list)) {
 		compat_exit_robust_list(tsk);
 		tsk->compat_robust_list = NULL;
+		exit_pi_state_list(tsk);
 	}
 #endif
-	if (unlikely(!list_empty(&tsk->pi_state_list)))
-		exit_pi_state_list(tsk);
 #endif
 
 	uprobe_free_utask(tsk);
-- 
2.15.2