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=-0.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no 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 C8848C35254 for ; Mon, 10 Feb 2020 04:28:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9CA6D214DB for ; Mon, 10 Feb 2020 04:28:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581308922; bh=l1OeRezZzhe+y0mBNHwN5J5otTAGBbttv+vIF5SGOJk=; h=Date:From:To:Cc:Subject:In-Reply-To:References:List-ID:From; b=xGS20uRMsblODeLKSyGVkm7v6gYdK8yxSK1vx0rRZaJckkr8izxIy4p/SUlt05DcK JmOzhmAlI3HEEf6H/p4hndcvwOESJQOTds9gUPnG/D5fLR+2mGaf/1ZII82un8+LcP RWXt9EvSORYRmZJeAaprA9Ylt22lVjWLKZJyyU+8= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727430AbgBJE2m (ORCPT ); Sun, 9 Feb 2020 23:28:42 -0500 Received: from mail.kernel.org ([198.145.29.99]:45206 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726950AbgBJE2l (ORCPT ); Sun, 9 Feb 2020 23:28:41 -0500 Received: from localhost.localdomain (c-73-231-172-41.hsd1.ca.comcast.net [73.231.172.41]) (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 7D748214DB; Mon, 10 Feb 2020 04:28:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581308920; bh=l1OeRezZzhe+y0mBNHwN5J5otTAGBbttv+vIF5SGOJk=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=16CmDXbCPWS2GhluQtjfPjZiGcRa6DeS3CtEcGMYB8mhvGi20OtXYMbMkIR31W+2C 5zAWDQKxBkgmbnZRu1EFfvvbU9j9vaofeckw1R3rowBSLuusqh01YIjP8JbNd3cLww 44VYaNI4cEWAIktNi/gYtd6WzPyBlKFY/59jKcLo= Date: Sun, 9 Feb 2020 20:28:40 -0800 From: Andrew Morton To: Qian Cai Cc: hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/memcontrol: fix a data race in scan count Message-Id: <20200209202840.2bf97ffcfa811550d733c461@linux-foundation.org> In-Reply-To: <20200206034945.2481-1-cai@lca.pw> References: <20200206034945.2481-1-cai@lca.pw> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 5 Feb 2020 22:49:45 -0500 Qian Cai wrote: > struct mem_cgroup_per_node mz.lru_zone_size[zone_idx][lru] could be > accessed concurrently as noticed by KCSAN, > > ... > > Reported by Kernel Concurrency Sanitizer on: > CPU: 95 PID: 50964 Comm: cc1 Tainted: G W O L 5.5.0-next-20200204+ #6 > Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 07/10/2019 > > The write is under lru_lock, but the read is done as lockless. The scan > count is used to determine how aggressively the anon and file LRU lists > should be scanned. Load tearing could generate an inefficient heuristic, > so fix it by adding READ_ONCE() for the read. > > ... > > --- a/include/linux/memcontrol.h > +++ b/include/linux/memcontrol.h > @@ -533,7 +533,7 @@ unsigned long mem_cgroup_get_zone_lru_size(struct lruvec *lruvec, > struct mem_cgroup_per_node *mz; > > mz = container_of(lruvec, struct mem_cgroup_per_node, lruvec); > - return mz->lru_zone_size[zone_idx][lru]; > + return READ_ONCE(mz->lru_zone_size[zone_idx][lru]); > } I worry about the readability/maintainability of these things. A naive reader who comes upon this code will wonder "why the heck is it using READ_ONCE?". A possibly lengthy trawl through the git history will reveal the reason but that's rather unkind. Wouldn't a simple /* modified under lru_lock, so use READ_ONCE */ improve the situation? From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: Re: [PATCH] mm/memcontrol: fix a data race in scan count Date: Sun, 9 Feb 2020 20:28:40 -0800 Message-ID: <20200209202840.2bf97ffcfa811550d733c461@linux-foundation.org> References: <20200206034945.2481-1-cai@lca.pw> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581308920; bh=l1OeRezZzhe+y0mBNHwN5J5otTAGBbttv+vIF5SGOJk=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=16CmDXbCPWS2GhluQtjfPjZiGcRa6DeS3CtEcGMYB8mhvGi20OtXYMbMkIR31W+2C 5zAWDQKxBkgmbnZRu1EFfvvbU9j9vaofeckw1R3rowBSLuusqh01YIjP8JbNd3cLww 44VYaNI4cEWAIktNi/gYtd6WzPyBlKFY/59jKcLo= In-Reply-To: <20200206034945.2481-1-cai-J5quhbR+WMc@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" To: Qian Cai Cc: hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org, mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, vdavydov.dev-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org On Wed, 5 Feb 2020 22:49:45 -0500 Qian Cai wrote: > struct mem_cgroup_per_node mz.lru_zone_size[zone_idx][lru] could be > accessed concurrently as noticed by KCSAN, > > ... > > Reported by Kernel Concurrency Sanitizer on: > CPU: 95 PID: 50964 Comm: cc1 Tainted: G W O L 5.5.0-next-20200204+ #6 > Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 07/10/2019 > > The write is under lru_lock, but the read is done as lockless. The scan > count is used to determine how aggressively the anon and file LRU lists > should be scanned. Load tearing could generate an inefficient heuristic, > so fix it by adding READ_ONCE() for the read. > > ... > > --- a/include/linux/memcontrol.h > +++ b/include/linux/memcontrol.h > @@ -533,7 +533,7 @@ unsigned long mem_cgroup_get_zone_lru_size(struct lruvec *lruvec, > struct mem_cgroup_per_node *mz; > > mz = container_of(lruvec, struct mem_cgroup_per_node, lruvec); > - return mz->lru_zone_size[zone_idx][lru]; > + return READ_ONCE(mz->lru_zone_size[zone_idx][lru]); > } I worry about the readability/maintainability of these things. A naive reader who comes upon this code will wonder "why the heck is it using READ_ONCE?". A possibly lengthy trawl through the git history will reveal the reason but that's rather unkind. Wouldn't a simple /* modified under lru_lock, so use READ_ONCE */ improve the situation?