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=-9.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 8B130C4727C for ; Tue, 22 Sep 2020 01:25:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4EE3020709 for ; Tue, 22 Sep 2020 01:25:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729772AbgIVBZ4 (ORCPT ); Mon, 21 Sep 2020 21:25:56 -0400 Received: from mail.kernel.org ([198.145.29.99]:56614 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729558AbgIVBYx (ORCPT ); Mon, 21 Sep 2020 21:24:53 -0400 Received: from gandalf.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (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 E499723AC6; Tue, 22 Sep 2020 01:24:52 +0000 (UTC) Received: from rostedt by gandalf.local.home with local (Exim 4.94) (envelope-from ) id 1kKX35-001sMg-Vi; Mon, 21 Sep 2020 21:24:51 -0400 Message-ID: <20200922012451.863339519@goodmis.org> User-Agent: quilt/0.66 Date: Mon, 21 Sep 2020 21:24:27 -0400 From: Steven Rostedt To: linux-kernel@vger.kernel.org Cc: Ingo Molnar , Andrew Morton , Oleg Nesterov , Davidlohr Bueso Subject: [for-next][PATCH 13/26] fgraph: Convert ret_stack tasklist scanning to rcu References: <20200922012414.115238201@goodmis.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Davidlohr Bueso It seems that alloc_retstack_tasklist() can also take a lockless approach for scanning the tasklist, instead of using the big global tasklist_lock. For this we also kill another deprecated and rcu-unsafe tsk->thread_group user replacing it with for_each_process_thread(), maintaining semantics. Here tasklist_lock does not protect anything other than the list against concurrent fork/exit. And considering that the whole thing is capped by FTRACE_RETSTACK_ALLOC_SIZE (32), it should not be a problem to have a pontentially stale, yet stable, list. The task cannot go away either, so we don't risk racing with ftrace_graph_exit_task() which clears the retstack. The tsk->ret_stack management is not protected by tasklist_lock, being serialized with the corresponding publish/subscribe barriers against concurrent ftrace_push_return_trace(). In addition this plays nicer with cachelines by avoiding two atomic ops in the uncontended case. Link: https://lkml.kernel.org/r/20200907013326.9870-1-dave@stgolabs.net Acked-by: Oleg Nesterov Signed-off-by: Davidlohr Bueso Signed-off-by: Steven Rostedt (VMware) --- kernel/trace/fgraph.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c index 1af321dec0f1..5658f13037b3 100644 --- a/kernel/trace/fgraph.c +++ b/kernel/trace/fgraph.c @@ -387,8 +387,8 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list) } } - read_lock(&tasklist_lock); - do_each_thread(g, t) { + rcu_read_lock(); + for_each_process_thread(g, t) { if (start == end) { ret = -EAGAIN; goto unlock; @@ -403,10 +403,10 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list) smp_wmb(); t->ret_stack = ret_stack_list[start++]; } - } while_each_thread(g, t); + } unlock: - read_unlock(&tasklist_lock); + rcu_read_unlock(); free: for (i = start; i < end; i++) kfree(ret_stack_list[i]); -- 2.28.0