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Subject[PATCH] futex: robust futex maybe never be awaked, on rare situation.
Date
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@zte.com.cn>
Reviewed-by: Bo Wang <wang.bo116@zte.com.cn>
Reviewed-by: Yi Wang <wang.yi59@zte.com.cn>
---
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
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