On 05/12, Davidlohr Bueso wrote: > > On Tue, 12 May 2020, Oleg Nesterov wrote: > > >do_each_pid_task(PIDTYPE_PGID) can race with change_pid(PIDTYPE_PGID) > >which moves the task from one hlist to another. Yes, it is safe in > >that task_struct can't go away. But still this is not right because > >do_each_pid_task() can scan the wrong (2nd) hlist. > > Hmm I didn't think about this case, I guess this is also busted in > ioprio_get(2) then.
agreed... > > > >could you explain in details why do you think this PF_EXITING check > >makes any sense? > > My logic was that if the task with the highest prio exited while we > were iterating the list, it would not be necessarily seen with rcu > and the syscall would return the highest prio of a task that exited; > and checking against PF_EXITING was a way to ignore such scenarios > as we were going to race with it anyway. Sorry, still can't understand. The PF_EXITING flag is not protected by tasklist_lock or rcu_lock. OK, if nothing else. Suppose that a prgp has a single process P, this proces has already exited but its parent didn't do wait(). Currently getpriority() returns task_nice(P). With the PF_EXITING check it will return -ESRCH. Hmm? Oleg.
