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=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,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 585E4C48BD1 for ; Fri, 11 Jun 2021 08:59:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3A865613CA for ; Fri, 11 Jun 2021 08:59:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231463AbhFKJBR (ORCPT ); Fri, 11 Jun 2021 05:01:17 -0400 Received: from mail.kernel.org ([198.145.29.99]:60262 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231458AbhFKJBO (ORCPT ); Fri, 11 Jun 2021 05:01:14 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 0B4A660BD3; Fri, 11 Jun 2021 08:59:15 +0000 (UTC) Date: Fri, 11 Jun 2021 09:59:13 +0100 From: Catalin Marinas To: Yanfei Xu Cc: akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/kmemleak: use READ_ONCE() for accessing jiffies_scan_wait Message-ID: <20210611085913.GA8132@arm.com> References: <20210609155657.26972-1-yanfei.xu@windriver.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210609155657.26972-1-yanfei.xu@windriver.com> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 09, 2021 at 11:56:57PM +0800, Yanfei Xu wrote: > The stop_scan_thread() and start_scan_thread() cannot really solve > the problem of concurrent accessing the global jiffies_scan_wait. > > kmemleak_write kmemleak_scan_thread > while (!kthread_should_stop()) > stop_scan_thread > jiffies_scan_wait = xxx timeout = jiffies_scan_wait > start_scan_thread > > We could replace these with a READ_ONCE() when reading > jiffies_scan_wait. It also can prevent compiler from reordering the > jiffies_scan_wait which is in while loop. I'm ok with READ_ONCE but your patch introduces functional changes. > diff --git a/mm/kmemleak.c b/mm/kmemleak.c > index 92a2d4885808..5ccf3969b7fe 100644 > --- a/mm/kmemleak.c > +++ b/mm/kmemleak.c > @@ -1567,7 +1567,7 @@ static int kmemleak_scan_thread(void *arg) > } > > while (!kthread_should_stop()) { > - signed long timeout = jiffies_scan_wait; > + signed long timeout = READ_ONCE(jiffies_scan_wait); > > mutex_lock(&scan_mutex); > kmemleak_scan(); > @@ -1812,11 +1812,8 @@ static ssize_t kmemleak_write(struct file *file, const char __user *user_buf, > ret = kstrtoul(buf + 5, 0, &secs); > if (ret < 0) > goto out; > - stop_scan_thread(); > - if (secs) { > + if (secs) > jiffies_scan_wait = msecs_to_jiffies(secs * 1000); For symmetry, I'd add a WRITE_ONCE here as well. > - start_scan_thread(); > - } The reason for stop/start_scan_thread() wasn't to protect against jiffies_scan_wait access but rather to force a new delay. Let's say you start by default with a 10min delay between scans (default) but you want to lower it to 1min. With the above removal of stop/start, you'd still have to wait for 10min until the scanning thread will notice the change. Also, with secs=0, the expectations is that the thread won't be restarted but this is removed by your patch. -- Catalin