Messages in this thread | | | Date | Tue, 12 May 2020 18:41:31 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 1/2] kernel/sys: only rely on rcu for getpriority(2) |
| |
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.
| |