Messages in this thread | | | Date | Wed, 21 Aug 2019 15:31:39 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [patch 04/44] posix-cpu-timers: Fixup stale comment |
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On Wed, 21 Aug 2019, Frederic Weisbecker wrote: > On Tue, Aug 20, 2019 at 11:43:26PM +0200, Thomas Gleixner wrote: > > On Tue, 20 Aug 2019, Frederic Weisbecker wrote: > > 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. > > That's exactly what I'm proposing. I think you're misunderstanding me. > > I want the owner to handle all the list deletion work from the target. > > Ok let's imagine a timer $ITIMER, owned by task $OWNER and whose target is task $TARGET. > > So it's enqueued on $OWNER->signal->posix_timers and $TARGET->cputime_expires. > > Two scenarios can happen: > > 1) $TARGET exits first and is released. So it calls posix_cpu_timers_exit() > which deletes $ITIMER from $TARGET->cputime_expires. > > Later on, $OWNER exits and calls exit_itimers() -> timer_delete_hook($ITIMER) > -> posix_cpu_timer_del($ITIMER). It finds $TARGET as the target of $ITIMER but no > more sighand. So it returns. > > 2) $OWNER exits first and calls exit_itimer() -> timer_delete_hook($ITIMER) > -> posix_cpu_timer_del($ITIMER). It finds $TARGET as the target of $ITIMER and it > finds a sighand to lock. So it deletes $ITIMER from $TARGET->cputime_expires > (see list_del(&timer->it.cpu.entry)). > > > So I propose to change the behaviour of case 1) so that $TARGET doesn't call > posix_cpu_timers_exit(). We instead wait for $OWNER to exit and call > exit_itimers() -> timer_delete_hook($ITIMER) -> posix_cpu_timer_del($ITIMER). > It is going to find $TARGET as the target of $ITIMER but no more sighand. Then > finally it removes $ITIMER from $TARGET->cputime_expires. > We basically do the same thing as in 2) but without locking sighand since it's NULL > on $TARGET at this time.
But what do we win with that? Horrors like this:
task A task B task C
arm_timer(A) arm_timer(A)
do_exit()
del_timer(A) del_timer(A) no sighand no_sighand list_del() list_del()
Guess how well concurrent list deletion works.
We must remove armed timers from the task/signal _before_ dropping sighand, really.
Thanks,
tglx
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