From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755646AbaHATAv (ORCPT ); Fri, 1 Aug 2014 15:00:51 -0400 Received: from mx1.redhat.com ([209.132.183.28]:9446 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750851AbaHATAu (ORCPT ); Fri, 1 Aug 2014 15:00:50 -0400 Date: Fri, 1 Aug 2014 20:57:53 +0200 From: Oleg Nesterov To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, bobby.prani@gmail.com Subject: Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks() Message-ID: <20140801185753.GC10718@redhat.com> References: <20140731215445.GA21933@linux.vnet.ibm.com> <1406843709-23396-1-git-send-email-paulmck@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1406843709-23396-1-git-send-email-paulmck@linux.vnet.ibm.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/31, Paul E. McKenney wrote: > > + rcu_read_lock(); > + for_each_process_thread(g, t) { > + if (t != current && ACCESS_ONCE(t->on_rq) && > + !is_idle_task(t)) { > + get_task_struct(t); > + t->rcu_tasks_nvcsw = ACCESS_ONCE(t->nvcsw); > + ACCESS_ONCE(t->rcu_tasks_holdout) = 1; > + list_add(&t->rcu_tasks_holdout_list, > + &rcu_tasks_holdouts); > + } > + } > + rcu_read_unlock(); So let me repeat. for_each_process_thread() above can not (in general) see the exiting tasks which have already called exit_notify(), because such a task can be removed from rcu lists at any time. Now suppose that proc_exit_connector() is probed. Or another function which can be called after exit_notify(), this doesn't matter. An exiting task T jumps into trampoline and gets the preemption. It can be already (auto)reaped and removed from rcu lists. synchronize_rcu_tasks() can not see this task, so it can return before T gets a chance to resume. Oleg.