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 AD53DC47404 for ; Mon, 7 Oct 2019 14:30:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8F8FD2190F for ; Mon, 7 Oct 2019 14:30:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728073AbfJGOaF (ORCPT ); Mon, 7 Oct 2019 10:30:05 -0400 Received: from mx2.suse.de ([195.135.220.15]:40598 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726334AbfJGOaF (ORCPT ); Mon, 7 Oct 2019 10:30:05 -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 D9871AF5B; Mon, 7 Oct 2019 14:30:02 +0000 (UTC) Date: Mon, 7 Oct 2019 16:30:02 +0200 From: Petr Mladek To: Qian Cai Cc: akpm@linux-foundation.org, sergey.senozhatsky.work@gmail.com, rostedt@goodmis.org, peterz@infradead.org, mhocko@kernel.org, linux-mm@kvack.org, john.ogness@linutronix.de, david@redhat.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk() Message-ID: <20191007143002.l37bt2lzqtnqjqxu@pathway.suse.cz> References: <1570228005-24979-1-git-send-email-cai@lca.pw> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1570228005-24979-1-git-send-email-cai@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 Fri 2019-10-04 18:26:45, Qian Cai wrote: > It is unsafe to call printk() while zone->lock was held, i.e., > > zone->lock --> console_lock > > because the console could always allocate some memory in different code > paths and form locking chains in an opposite order, > > console_lock --> * --> zone->lock > > As the result, it triggers lockdep splats like below and in different > code paths in this thread [1]. Since has_unmovable_pages() was only used > in set_migratetype_isolate() and is_pageblock_removable_nolock(). Only > the former will set the REPORT_FAILURE flag which will call printk(). > Hence, unlock the zone->lock just before the dump_page() there where > when has_unmovable_pages() returns true, there is no need to hold the > lock anyway in the rest of set_migratetype_isolate(). > > While at it, remove a problematic printk() in __offline_isolated_pages() > only for debugging as well which will always disable lockdep on debug > kernels. > > The problem is probably there forever, but neither many developers will > run memory offline with the lockdep enabled nor admins in the field are > lucky enough yet to hit a perfect timing which required to trigger a > real deadlock. In addition, there aren't many places that call printk() > while zone->lock was held. > > WARNING: possible circular locking dependency detected > ------------------------------------------------------ > test.sh/1724 is trying to acquire lock: > 0000000052059ec0 (console_owner){-...}, at: console_unlock+0x > 01: 328/0xa30 > > but task is already holding lock: > 000000006ffd89c8 (&(&zone->lock)->rlock){-.-.}, at: start_iso > 01: late_page_range+0x216/0x538 > > which lock already depends on the new lock. > > the existing dependency chain (in reverse order) is: > > -> #2 (&(&zone->lock)->rlock){-.-.}: > lock_acquire+0x21a/0x468 > _raw_spin_lock+0x54/0x68 > get_page_from_freelist+0x8b6/0x2d28 > __alloc_pages_nodemask+0x246/0x658 > __get_free_pages+0x34/0x78 > sclp_init+0x106/0x690 > sclp_register+0x2e/0x248 > sclp_rw_init+0x4a/0x70 > sclp_console_init+0x4a/0x1b8 > console_init+0x2c8/0x410 > start_kernel+0x530/0x6a0 > startup_continue+0x70/0xd0 This code takes locks: sclp_lock --> &(&zone->lock)->rlock > -> #1 (sclp_lock){-.-.}: > lock_acquire+0x21a/0x468 > _raw_spin_lock_irqsave+0xcc/0xe8 > sclp_add_request+0x34/0x308 > sclp_conbuf_emit+0x100/0x138 > sclp_console_write+0x96/0x3b8 > console_unlock+0x6dc/0xa30 > vprintk_emit+0x184/0x3c8 > vprintk_default+0x44/0x50 > printk+0xa8/0xc0 > iommu_debugfs_setup+0xf2/0x108 > iommu_init+0x6c/0x78 > do_one_initcall+0x162/0x680 > kernel_init_freeable+0x4e8/0x5a8 > kernel_init+0x2a/0x188 > ret_from_fork+0x30/0x34 > kernel_thread_starter+0x0/0xc This code path takes: console_owner --> sclp_lock > -> #0 (console_owner){-...}: > check_noncircular+0x338/0x3e0 > __lock_acquire+0x1e66/0x2d88 > lock_acquire+0x21a/0x468 > console_unlock+0x3a6/0xa30 > vprintk_emit+0x184/0x3c8 > vprintk_default+0x44/0x50 > printk+0xa8/0xc0 > __dump_page+0x1dc/0x710 > dump_page+0x2e/0x58 > has_unmovable_pages+0x2e8/0x470 > start_isolate_page_range+0x404/0x538 > __offline_pages+0x22c/0x1338 > memory_subsys_offline+0xa6/0xe8 > device_offline+0xe6/0x118 > state_store+0xf0/0x110 > kernfs_fop_write+0x1bc/0x270 > vfs_write+0xce/0x220 > ksys_write+0xea/0x190 > system_call+0xd8/0x2b4 And this code path takes: &(&zone->lock)->rlock --> console_owner > other info that might help us debug this: > > Chain exists of: > console_owner --> sclp_lock --> &(&zone->lock)->rlock All three code paths together create a cyclic dependency. This is why lockdep reports a possible deadlock. I believe that it cannot really happen because: static int __init sclp_console_init(void) { [...] rc = sclp_rw_init(); [...] register_console(&sclp_console); return 0; } sclp_rw_init() is called before register_console(). And console_unlock() will never call sclp_console_write() before the console is registered. AFAIK, lockdep only compares existing chain of locks. It does not know about console registration that would make some code paths mutually exclusive. I believe that it is a false positive. I do not know how to avoid this lockdep report. I hope that it will disappear by deferring all printk() calls rather soon. Best Regards, Petr