So I still think the pid_alive() check should die... and when I look at this code again I don't understand why does it abuse task_struct->usage, I'll send another patch on top of this one. On 08/21, Oleg Nesterov wrote: > > On 08/21, Kui-Feng Lee wrote: > > > > > > On 8/21/23 08:09, Oleg Nesterov wrote: > > >1. find_pid_ns() + get_pid_task() under rcu_read_lock() guarantees that we > > > can safely iterate the task->thread_group list. Even if this task exits > > > right after get_pid_task() (or goto retry) and pid_alive() returns 0 > > > > Kill the unnecessary pid_alive() check. > > > > This function will return next_task holding a refcount, and release the > > refcount until the next time calling the same function. Meanwhile, > > the returned task A may be killed, and its next task B may be > > killed after A as well, before calling this function again. > > However, even task B is destroyed (free), A's next is still pointing to > > task B. When this function is called again for the same iterator, > > it doesn't promise that B is still there. > > Not sure I understand... > > OK, if we have a task pointer with incremented refcount and do not hold > rcu lock, then yes, you can't remove the pid_alive() check in this code: > > rcu_read_lock(); > if (pid_alive(task)) > do_something(next_thread(task)); > rcu_read_unlock(); > > because task and then task->next can exit and do call_rcu(delayed_put_task_struct) > before we take rcu_read_lock(). > > But if you do something like > > rcu_read_lock(); > > task = find_task_in_some_rcu_protected_list(); > do_something(next_thread(task)); > > rcu_read_unlock(); > > then next_thread(task) should be safe without pid_alive(). > > And iiuc task_group_seq_get_next() always does > > rcu_read_lock(); // the caller does lock/unlock > > task = get_pid_task(pid, PIDTYPE_PID); > if (!task) > return; > > next_task = next_thread(task); > > rcu_read_unlock(); > > Yes, both task and task->next can exit right after get_pid_task(), but since > can only happen after we took rcu_read_lock(), delayed_put_task_struct() can't > be called until we drop rcu lock. > > What have I missed? > > Oleg.
