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 54D3DC4360C for ; Fri, 4 Oct 2019 08:14:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1D7882084D for ; Fri, 4 Oct 2019 08:14:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1570176856; bh=8bLGvd62yyXJEEM75akm9KRsOhNqib/V80CUazhK6OE=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=J6EBhbm2XpFUs9GcNX8nsWq8eHvypFdCIIrPU1dkBL4jF745HXACd4md/9NuuvNhR MnYP6QQvFb1xhFi+dViE1UF9v8qiC/uxp0XdLOBm3Eqi+Kijr4bnIwEXxfmkKDYbdC MmZ64QSaHpKr5YT3toosi4B6IV9i87vXq9R1hehQ= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387801AbfJDIOP (ORCPT ); Fri, 4 Oct 2019 04:14:15 -0400 Received: from mx2.suse.de ([195.135.220.15]:34936 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S2387631AbfJDIOP (ORCPT ); Fri, 4 Oct 2019 04:14:15 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 6495EB16B; Fri, 4 Oct 2019 08:14:12 +0000 (UTC) Date: Fri, 4 Oct 2019 10:13:58 +0200 From: Michal Hocko To: Qian Cai Cc: akpm@linux-foundation.org, cl@linux.com, penberg@kernel.org, rientjes@google.com, tj@kernel.org, vdavydov.dev@gmail.com, hannes@cmpxchg.org, guro@fb.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/slub: fix a deadlock in show_slab_objects() Message-ID: <20191004081358.GA9578@dhcp22.suse.cz> References: <1570131869-2545-1-git-send-email-cai@lca.pw> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1570131869-2545-1-git-send-email-cai@lca.pw> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 03-10-19 15:44:29, Qian Cai wrote: > Long time ago, there fixed a similar deadlock in show_slab_objects() > [1]. However, it is apparently due to the commits like 01fb58bcba63 > ("slab: remove synchronous synchronize_sched() from memcg cache > deactivation path") and 03afc0e25f7f ("slab: get_online_mems for > kmem_cache_{create,destroy,shrink}"), this kind of deadlock is back by > just reading files in /sys/kernel/slab will generate a lockdep splat > below. > > Since the "mem_hotplug_lock" here is only to obtain a stable online node > mask while racing with NUMA node hotplug, it is probably fine to do > without it. "It is probably fine" is not a proper justification. Please have a look at my older email where I've exaplained why I believe it is safe. > WARNING: possible circular locking dependency detected > ------------------------------------------------------ I pressume the deadlock is real. If that is the case then Cc: stable and Fixes tag would be really appreciated. > Signed-off-by: Qian Cai Anyway, I do agree that this is the right thing to do. With the improved changelog, fixed up the comment alignment feel free to add Acked-by: Michal Hocko > --- > mm/slub.c | 11 +++++++++-- > 1 file changed, 9 insertions(+), 2 deletions(-) > > diff --git a/mm/slub.c b/mm/slub.c > index 42c1b3af3c98..922cdcf5758a 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -4838,7 +4838,15 @@ static ssize_t show_slab_objects(struct kmem_cache *s, > } > } > > - get_online_mems(); > +/* > + * It is not possible to take "mem_hotplug_lock" here, as it has already held > + * "kernfs_mutex" which could race with the lock order: > + * > + * mem_hotplug_lock->slab_mutex->kernfs_mutex > + * > + * In the worest case, it might be mis-calculated while doing NUMA node > + * hotplug, but it shall be corrected by later reads of the same files. > + */ > #ifdef CONFIG_SLUB_DEBUG > if (flags & SO_ALL) { > struct kmem_cache_node *n; > @@ -4879,7 +4887,6 @@ static ssize_t show_slab_objects(struct kmem_cache *s, > x += sprintf(buf + x, " N%d=%lu", > node, nodes[node]); > #endif > - put_online_mems(); > kfree(nodes); > return x + sprintf(buf + x, "\n"); > } > -- > 1.8.3.1 > -- Michal Hocko SUSE Labs