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=-5.3 required=3.0 tests=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 20655C10F14 for ; Tue, 8 Oct 2019 07:44:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 006E021721 for ; Tue, 8 Oct 2019 07:44:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730405AbfJHHoA (ORCPT ); Tue, 8 Oct 2019 03:44:00 -0400 Received: from mx2.suse.de ([195.135.220.15]:39908 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1730222AbfJHHn7 (ORCPT ); Tue, 8 Oct 2019 03:43:59 -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 0DF5FAB98; Tue, 8 Oct 2019 07:43:58 +0000 (UTC) Date: Tue, 8 Oct 2019 09:43:57 +0200 From: Petr Mladek To: Michal Hocko Cc: Qian Cai , akpm@linux-foundation.org, sergey.senozhatsky.work@gmail.com, rostedt@goodmis.org, peterz@infradead.org, linux-mm@kvack.org, john.ogness@linutronix.de, david@redhat.com, linux-kernel@vger.kernel.org, Heiko Carstens , Vasily Gorbik , Christian Borntraeger Subject: Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk() Message-ID: <20191008074357.f33f6pbs4cw5majk@pathway.suse.cz> References: <1570228005-24979-1-git-send-email-cai@lca.pw> <20191007143002.l37bt2lzqtnqjqxu@pathway.suse.cz> <20191007144937.GO2381@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191007144937.GO2381@dhcp22.suse.cz> 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 16:49:37, Michal Hocko wrote: > [Cc s390 maintainers - the lockdep is http://lkml.kernel.org/r/1570228005-24979-1-git-send-email-cai@lca.pw > Petr has explained it is a false positive > http://lkml.kernel.org/r/20191007143002.l37bt2lzqtnqjqxu@pathway.suse.cz] > On Mon 07-10-19 16:30:02, Petr Mladek wrote: > [...] > > 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. > > Thanks a lot for looking into this Petr. I have also checked the code > and I really fail to see why the allocation has to be done under the > lock in the first place. sclp_read_sccb and sclp_init_sccb are global > variables but I strongly suspect that they need a synchronization during > early init, callbacks are registered only later IIUC: Good idea. It would work when the init function is called only once. But see below. > diff --git a/drivers/s390/char/sclp.c b/drivers/s390/char/sclp.c > index d2ab3f07c008..4b1c033e3255 100644 > --- a/drivers/s390/char/sclp.c > +++ b/drivers/s390/char/sclp.c > @@ -1169,13 +1169,13 @@ sclp_init(void) > unsigned long flags; > int rc = 0; > > + sclp_read_sccb = (void *) __get_free_page(GFP_ATOMIC | GFP_DMA); > + sclp_init_sccb = (void *) __get_free_page(GFP_ATOMIC | GFP_DMA); > spin_lock_irqsave(&sclp_lock, flags); > /* Check for previous or running initialization */ > if (sclp_init_state != sclp_init_state_uninitialized) > goto fail_unlock; It seems that sclp_init() could be called several times in parallel. I see it called from sclp_register() and sclp_initcall(). I am not sure if it is really needed or if it is just a strange desing. It might be still possible to always do the allocation without the lock and free the memory when it is not really used. But I am not sure if we want to do this exercise just to avoid lockdep false positive. Best Regards, Petr