lkml.org 
[lkml]   [2019]   [Aug]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [patch 04/44] posix-cpu-timers: Fixup stale comment
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.

I hope I'm less confusing (if not confused).

Thanks.

\
 
 \ /
  Last update: 2019-08-21 00:56    [W:0.147 / U:0.336 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site