From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756789Ab0FKRhu (ORCPT ); Fri, 11 Jun 2010 13:37:50 -0400 Received: from mx1.redhat.com ([209.132.183.28]:4273 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752950Ab0FKRht (ORCPT ); Fri, 11 Jun 2010 13:37:49 -0400 Date: Fri, 11 Jun 2010 18:40:51 +0200 From: Stanislaw Gruszka To: Oleg Nesterov Cc: Ingo Molnar , Peter Zijlstra , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: Re: [PATCH 4/5] thread_group_cputime: simplify, document the "alive" check Message-ID: <20100611164050.GA19325@dhcp-lab-161.englab.brq.redhat.com> References: <20100610230956.GA25921@redhat.com> <20100611143525.GA2586@dhcp-lab-161.englab.brq.redhat.com> <20100611151533.GA2867@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100611151533.GA2867@redhat.com> User-Agent: Mutt/1.5.19 (2009-01-05) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jun 11, 2010 at 05:15:33PM +0200, Oleg Nesterov wrote: > On 06/11, Stanislaw Gruszka wrote: > > > > On Fri, Jun 11, 2010 at 01:09:56AM +0200, Oleg Nesterov wrote: > > > thread_group_cputime() looks as if it is rcu-safe, but in fact this > > > was wrong until ea6d290c which pins task->signal to task_struct. > > > It checks ->sighand != NULL under rcu, but this can't help if ->signal > > > can go away. Fortunately the caller either holds ->siglock, or it is > > > fastpath_timer_check() which uses current and checks exit_state == 0. > > > > Hmm, I thought we avoided calling thread_group_cputime() from > > fastpatch_timer_check(), but seems it is still possible when we > > call run_posix_cpu_timers() on two different cpus simultaneously ... > > No, we can't. thread_group_cputimer() does test-and-set ->running > under cputimer->lock. > > But when I sent these patches, I realized we have another race here > (with or without these patches). I am already doing the fix. Don't know what you catch, I was thinking about: cpu0 cpu1 fastpath_timer_check(): if (sig->cputimer.running) { struct task_cputime group_sample; stop_process_timers(): spin_lock_irqsave(&cputimer->lock, flags); cputimer->running = 0; spin_unlock_irqrestore(&cputimer->lock, flags); thread_group_cputimer(tsk, &group_sample); > > > - Since ea6d290c commit tsk->signal is stable, we can read it first > > > and avoid the initialization from INIT_CPUTIME. > > > > > > - Even if tsk->signal is always valid, we still have to check it > > > is safe to use next_thread() under rcu_read_lock(). Currently > > > the code checks ->sighand != NULL, change it to use pid_alive() > > > which is commonly used to ensure the task wasn't unhashed before > > > we take rcu_read_lock(). > > > > I'm not sure how important are values of almost dead task, but > > perhaps would be better to return times form all threads > > using as base sig->curr_target in loop. > > Could you clarify? Avoid pid_alive check and loop starting from sig->curr_target: t = tsk = sig->curr_target; do { times->utime = cputime_add(times->utime, t->utime); times->stime = cputime_add(times->stime, t->stime); times->sum_exec_runtime += t->se.sum_exec_runtime; } while_each_thread(tsk, t); I don't know what are rules regarding accessing sig->curr_target, but if this is done under sighand->siglock we should be safe. Question if if we always have lock taken, we tried to assure that in the past, but if we really do? Stanislaw