Messages in this thread | | | Date | Tue, 20 Aug 2019 23:43:26 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [patch 04/44] posix-cpu-timers: Fixup stale comment |
| |
On Tue, 20 Aug 2019, Frederic Weisbecker wrote: > On Tue, Aug 20, 2019 at 07:57:37PM +0200, Thomas Gleixner wrote: > > On Tue, 20 Aug 2019, Frederic Weisbecker wrote: > > > On Mon, Aug 19, 2019 at 04:31:45PM +0200, Thomas Gleixner wrote: > > > > /* > > > > - * Clean out CPU timers still ticking when a thread exited. The task > > > > - * pointer is cleared, and the expiry time is replaced with the residual > > > > - * time for later timer_gettime calls to return. > > > > + * Clean out CPU timers which are still armed when a thread exits. The > > > > + * timers are only removed from the list. No other updates are done. The > > > > + * corresponding posix timers are still accessible, but cannot be rearmed. > > > > + * > > > > * This must be called with the siglock held. > > > > */ > > > > static void cleanup_timers(struct list_head *head) > > > > > > Indeed and I believe we could avoid that step. We remove the sighand at the same > > > time so those can't be accessed anymore anyway. > > > > > > exit_itimers() takes care of the last call release and could force remove from > > > the list (although it might be taken care of in your series, haven't checked yet): > > > > No. The posix timer is not necessarily owned by the exiting task or > > process. It can be owned by a different entity which has permissions, > > e.g. parent. > > > > So those are not in the posix timer list of the exiting task, which gets > > cleaned up in exit_itimers(). Those are in the list of the task which armed > > the timer. The timer is merily queued in the 'active timers' list of the > > exiting task and posix_cpu_timers_exit()/posix_cpu_timers_exit_group() > > remove it before the task/signal structs go away. > > Sure, I understand there's two distinct things here: the owner that queues > timers in owner->sig->posix_timers (cleaned in exit_itimers()) and the target that queues > in target->[signal->]cputime_expires (cleaned in posix_cpu_timers_exit[_group](). > > So I'm wondering why we bother with posix_cpu_timers_exit[_group]() at > all when exit_itimers() could handle the list deletion from > target->[signal]->cputime_expires throughout posix_cpu_timer_del() as it > already does on targets that still have their sighands.
No it can't do that throughout posix_cpu_timer_del() because exit_itimers() can only look at current->signal->posix_timers which does not contain the posix timers owned by a different task/process.
We could of course invoke posix_cpu_timers_exit() from exit_itimers() but does that buy anything?
> It would make things more simple to delete the timer off the target from > the same caller and place and we could remove posix_cpu_timers_exit*().
We can't. The foreign owned cpu timers are not in cur->signal->posix_timers so how should we invoke posix_cpu_timer_del() on them. Only the owner task can. The only thing the exiting task can do is to remove the foreign timer from it's expiry list which has nothing to do with cur->signal->posix_timers.
cur->signal->posix_timers only contains posix timers which are owned by current not those which are owned by a different task and armed on the exiting one.
exit_itimers() handles cur->signal->posix_timers, i.e. timers owned by current.
posix_cpu_timers_exit() handles timers enqueued on current, which are foreign owned timers because exit_itimers() removed those which were owned by current already.
posix_cpu_timers_exit_group() handles timers enqueued on current->signal, which are foreign owned timers because exit_itimers() removed those which were owned by current already.
> Or is there something I'm awkwardly missing as usual? :-)
I think so :)
Thanks,
tglx
| |