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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 CDF55ECE58C for ; Mon, 7 Oct 2019 14:11:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A5D9A206C2 for ; Mon, 7 Oct 2019 14:11:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728111AbfJGOLC (ORCPT ); Mon, 7 Oct 2019 10:11:02 -0400 Received: from mx2.suse.de ([195.135.220.15]:34056 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727745AbfJGOLC (ORCPT ); Mon, 7 Oct 2019 10:11:02 -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 999A6ADE0; Mon, 7 Oct 2019 14:11:00 +0000 (UTC) Date: Mon, 7 Oct 2019 16:10:59 +0200 From: Petr Mladek To: Qian Cai Cc: Michal Hocko , sergey.senozhatsky.work@gmail.com, rostedt@goodmis.org, peterz@infradead.org, linux-mm@kvack.org, john.ogness@linutronix.de, akpm@linux-foundation.org, david@redhat.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk() Message-ID: <20191007141059.friotqx2ymwvlo3j@pathway.suse.cz> References: <20191007080742.GD2381@dhcp22.suse.cz> <20191007113710.GH2381@dhcp22.suse.cz> <1570450304.5576.283.camel@lca.pw> <20191007124356.GJ2381@dhcp22.suse.cz> <1570453622.5576.288.camel@lca.pw> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1570453622.5576.288.camel@lca.pw> User-Agent: NeoMutt/20170912 (1.9.0) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 2019-10-07 09:07:02, Qian Cai wrote: > On Mon, 2019-10-07 at 14:43 +0200, Michal Hocko wrote: > > On Mon 07-10-19 08:11:44, Qian Cai wrote: > > > On Mon, 2019-10-07 at 13:37 +0200, Michal Hocko wrote: > > > > On Mon 07-10-19 07:04:00, Qian Cai wrote: > > > > > > > > > > > > > > > > On Oct 7, 2019, at 4:07 AM, Michal Hocko wrote: > > > > > > > > > > > > I do not think that removing the printk is the right long term solution. > > > > > > While I do agree that removing the debugging printk __offline_isolated_pages > > > > > > does make sense because it is essentially of a very limited use, this > > > > > > doesn't really solve the underlying problem. There are likely other > > > > > > printks from zone->lock. It would be much more saner to actually > > > > > > disallow consoles to allocate any memory while printk is called from an > > > > > > atomic context. > > > > > > > > > > No, there is only a handful of places called printk() from > > > > > zone->lock. It is normal that the callers will quietly process > > > > > “struct zone” modification in a short section with zone->lock > > > > > held. > > > > > > > > It is extremely error prone to have any zone->lock vs. printk > > > > dependency. I do not want to play an endless whack a mole. > > > > > > > > > No, it is not about “allocate any memory while printk is called from an > > > > > atomic context”. It is opposite lock chain from different processors which has the same effect. For example, > > > > > > > > > > CPU0: CPU1: CPU2: > > > > > console_owner > > > > > sclp_lock > > > > > sclp_lock zone_lock > > > > > zone_lock > > > > > console_owner > > > > > > > > Why would sclp_lock ever take a zone->lock (apart from an allocation). > > > > So really if sclp_lock is a lock that might be taken from many contexts > > > > and generate very subtle lock dependencies then it should better be > > > > really careful what it is calling into. > > > > > > > > In other words you are trying to fix a wrong end of the problem. Fix the > > > > console to not allocate or depend on MM by other means. > > > > > > It looks there are way too many places that could generate those indirect lock > > > chains that are hard to eliminate them all. Here is anther example, where it > > > has, > > > > Yeah and I strongly suspect they are consoles which are broken and need > > to be fixed rathert than the problem papered over. > > > > I do realize how tempting it is to remove all printks from the > > zone->lock but do realize that as soon as the allocator starts using any > > other locks then we are back to square one and the problem is there > > again. We would have to drop _all_ printks from any locked section in > > the allocator and I do not think this is viable. > > > > Really, the only way forward is to make these consoles be more careful > > of external dependencies. > > Even with the new printk() Petr proposed. There is no guarantee it will fix it > properly. It looks like just reduce the chance of this kind of deadlocks as it > may or may not call wake_up_klogd() in vprintk_emit() depends on timing. The chain below is wrong: > zone->lock > printk_deferred() > vprintk_emit() > wake_up_klogd() wake_up_klogd() calls irq_work_queue(). It queues the work for an interrupt handler and triggers the interrupt. > wake_up_klogd_work_func() > console_unlock() The work is done in the interrupt context. The interrupt could never be handled under zone->lock. So, printk_deferred() would help. But I do not think that it is really needed. I am going to answer the original mail with all the full lockdep report. Best Regards, Petr