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=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 DB998C4CEC7 for ; Sun, 15 Sep 2019 13:54:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AF2CD214D8 for ; Sun, 15 Sep 2019 13:54:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1568555683; bh=N0vWoqK+eDQvtTNXths4QzYT07cwK9BO7VguOvyN3r0=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=jd543qY1bezojT15LvQPiTY1fJGxqRfhu/32feJa4oW2ECRbEiSfJ/bdP3Lz7PkXS 9aWgJ1lt3yf3kgeUOhDRqmeLb68wPEBIiRhxRR2TeY1Wcg1YLCBEgt9L4EDDiVptaA IGYxMUpzYkUPcpDYhSFsyccsyLlV5Ig4f9PeR6aY= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387686AbfIONym (ORCPT ); Sun, 15 Sep 2019 09:54:42 -0400 Received: from mail.kernel.org ([198.145.29.99]:36864 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387483AbfIONym (ORCPT ); Sun, 15 Sep 2019 09:54:42 -0400 Received: from paulmck-ThinkPad-P72 (96-84-246-146-static.hfc.comcastbusiness.net [96.84.246.146]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id B6CC8214C6; Sun, 15 Sep 2019 13:54:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1568555680; bh=N0vWoqK+eDQvtTNXths4QzYT07cwK9BO7VguOvyN3r0=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=Loc1Z5rLWnYSj8zzYGBsi/p9aUYvgARlv8P6vpxIU4KXaPK5Mz/PPc7/XRozONJLi sw9S9byDp/rCq06YZOn59LC1aQHQlcSdVCyFgO0mDFZKcWZIDy7lLR8WYE1KiRjXHh cOotVmyqkpIJZJR3OWblI/V58XAUgZiwSlvRIwEs= Date: Sun, 15 Sep 2019 06:54:37 -0700 From: "Paul E. McKenney" To: "Eric W. Biederman" Cc: Peter Zijlstra , Oleg Nesterov , Russell King - ARM Linux admin , Chris Metcalf , Christoph Lameter , Kirill Tkhai , Mike Galbraith , Thomas Gleixner , Ingo Molnar , Linux List Kernel Mailing , Davidlohr Bueso , Linus Torvalds Subject: Re: [PATCH v2 1/4] task: Add a count of task rcu users Message-ID: <20190915135437.GI30224@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20190830160957.GC2634@redhat.com> <87o906wimo.fsf@x220.int.ebiederm.org> <20190902134003.GA14770@redhat.com> <87tv9uiq9r.fsf@x220.int.ebiederm.org> <87k1aqt23r.fsf_-_@x220.int.ebiederm.org> <87muf7f4bf.fsf_-_@x220.int.ebiederm.org> <87woebdplt.fsf_-_@x220.int.ebiederm.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87woebdplt.fsf_-_@x220.int.ebiederm.org> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Sep 14, 2019 at 07:33:34AM -0500, Eric W. Biederman wrote: > > Add a count of the number of rcu users (currently 1) of the task > struct so that we can later add the scheduler case and get rid of the > very subtle task_rcu_dereference, and just use rcu_dereference. > > As suggested by Oleg have the count overlap rcu_head so that no > additional space in task_struct is required. > > Inspired-by: Linus Torvalds > Inspired-by: Oleg Nesterov > Signed-off-by: "Eric W. Biederman" > --- > include/linux/sched.h | 5 ++++- > include/linux/sched/task.h | 1 + > kernel/exit.c | 7 ++++++- > kernel/fork.c | 7 +++---- > 4 files changed, 14 insertions(+), 6 deletions(-) > > diff --git a/include/linux/sched.h b/include/linux/sched.h > index 9f51932bd543..99a4518b9b17 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -1142,7 +1142,10 @@ struct task_struct { > > struct tlbflush_unmap_batch tlb_ubc; > > - struct rcu_head rcu; > + union { > + refcount_t rcu_users; > + struct rcu_head rcu; > + }; > > /* Cache last used pipe for splice(): */ > struct pipe_inode_info *splice_pipe; > diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h > index 0497091e40c1..4c44c37236b2 100644 > --- a/include/linux/sched/task.h > +++ b/include/linux/sched/task.h > @@ -116,6 +116,7 @@ static inline void put_task_struct(struct task_struct *t) > } > > struct task_struct *task_rcu_dereference(struct task_struct **ptask); > +void put_task_struct_rcu_user(struct task_struct *task); > > #ifdef CONFIG_ARCH_WANTS_DYNAMIC_TASK_STRUCT > extern int arch_task_struct_size __read_mostly; > diff --git a/kernel/exit.c b/kernel/exit.c > index 5b4a5dcce8f8..2e259286f4e7 100644 > --- a/kernel/exit.c > +++ b/kernel/exit.c > @@ -182,6 +182,11 @@ static void delayed_put_task_struct(struct rcu_head *rhp) > put_task_struct(tsk); > } > > +void put_task_struct_rcu_user(struct task_struct *task) > +{ > + if (refcount_dec_and_test(&task->rcu_users)) > + call_rcu(&task->rcu, delayed_put_task_struct); We "instantly" transition from the union being ->rcu_user to being ->rcu, so there needs to be some mechanism that has previously made sure that nothing is going to attempt to use ->rcu on this task. We cannot be relying solely on something like atomic_add_unless(), because call_rcu() will likely result in ->rcu being non-zero. > +} > > void release_task(struct task_struct *p) > { > @@ -222,7 +227,7 @@ void release_task(struct task_struct *p) > > write_unlock_irq(&tasklist_lock); > release_thread(p); > - call_rcu(&p->rcu, delayed_put_task_struct); > + put_task_struct_rcu_user(p); This, along with the pre-existing initialization of ->rcu_user to two below gives some hope, at least assuming that release_task() is invoked after no one can possibly try to increment ->rcu_user. And in v5.2, release_task() is invoked from these places: o de_thread(). On this one, I must defer to Oleg, Peter, and crew. It might be that the list removals while write-holding the tasklist_lock do the trick, but that assumes that this lock is involved in acquiring a reference. o find_child_reaper(). This is invoked via exit_notify() from do_exit(), just after the call to exit_tasks_rcu_start(). This is OK from a Tasks RCU perspective because it is using separate synchornization. Something earlier must prevent new ->rcu_user references. o wait_task_zombie(). Here is hoping that the check for EXIT_DEAD is helpful here. I am not seeing how this would be safe, but then again this is only the first patch. Plus there is much about this use case that I do not know. OK, on to the other patches... Thanx, Paul > > p = leader; > if (unlikely(zap_leader)) > diff --git a/kernel/fork.c b/kernel/fork.c > index 2852d0e76ea3..9f04741d5c70 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -900,10 +900,9 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node) > if (orig->cpus_ptr == &orig->cpus_mask) > tsk->cpus_ptr = &tsk->cpus_mask; > > - /* > - * One for us, one for whoever does the "release_task()" (usually > - * parent) > - */ > + /* One for the user space visible state that goes away when reaped. */ > + refcount_set(&tsk->rcu_users, 1); > + /* One for the rcu users, and one for the scheduler */ > refcount_set(&tsk->usage, 2); > #ifdef CONFIG_BLK_DEV_IO_TRACE > tsk->btrace_seq = 0; > -- > 2.21.0.dirty >