From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A28E5C3A589 for ; Tue, 20 Aug 2019 21:43:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 77D082332A for ; Tue, 20 Aug 2019 21:43:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730974AbfHTVnh (ORCPT ); Tue, 20 Aug 2019 17:43:37 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:53111 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730811AbfHTVnh (ORCPT ); Tue, 20 Aug 2019 17:43:37 -0400 Received: from p5de0b6c5.dip0.t-ipconnect.de ([93.224.182.197] helo=nanos) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1i0Bue-0006Zm-OE; Tue, 20 Aug 2019 23:43:32 +0200 Date: Tue, 20 Aug 2019 23:43:26 +0200 (CEST) From: Thomas Gleixner To: Frederic Weisbecker cc: LKML , Oleg Nesterov , Ingo Molnar , Peter Zijlstra , John Stultz , Frederic Weisbecker , Anna-Maria Behnsen Subject: Re: [patch 04/44] posix-cpu-timers: Fixup stale comment In-Reply-To: <20190820204803.GH2093@lenoir> Message-ID: References: <20190819143141.221906747@linutronix.de> <20190819143801.747233612@linutronix.de> <20190820142658.GG2093@lenoir> <20190820204803.GH2093@lenoir> User-Agent: Alpine 2.21 (DEB 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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