On Mon, Sep 23, 2019 at 11:18:20AM +0800, Yunfeng Cui wrote:
> 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!
Did you forget/overlook the pi_lock fiddling in do_exit() ? I'm thinking
that would make the above impossible.
> Signed-off-by: Yunfeng Cui <[email protected]>
> Reviewed-by: Bo Wang <[email protected]>
> Reviewed-by: Yi Wang <[email protected]>
> ---
> 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
I'm also thinking this breaks all sorts by not unconditionally calling
exit_pi_state_list(). Specifically, it could leak PI-state for !robust
futexes.
>
> uprobe_free_utask(tsk);
