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=-5.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 54FD8ECE58C for ; Mon, 7 Oct 2019 14:50:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1C1F521479 for ; Mon, 7 Oct 2019 14:50:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1570459833; bh=+L1hKJ2VTUqRsllRhayb9YKLbiRzxfqRmz/3qXSqkK8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=0iOXvLGERc7FBj9PxW2QCuJ6a9AsYUiRreBZmR4g6nMxibz0IPb/4rHBN3P4GEQTw CSRm+ZtJOuFNQWG3XR3bxYSwqgBT6zel1X9KqIaWBCD1m7kU/aN2YS0ufPaj3ph+hK tNZ3HpTJwmpH3JgCN5gXeEiqsMbIwY2b26MwYybU= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728706AbfJGOtp (ORCPT ); Mon, 7 Oct 2019 10:49:45 -0400 Received: from mx2.suse.de ([195.135.220.15]:51726 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728592AbfJGOtk (ORCPT ); Mon, 7 Oct 2019 10:49:40 -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 7523EAD45; Mon, 7 Oct 2019 14:49:38 +0000 (UTC) Date: Mon, 7 Oct 2019 16:49:37 +0200 From: Michal Hocko To: Petr Mladek 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: <20191007144937.GO2381@dhcp22.suse.cz> References: <1570228005-24979-1-git-send-email-cai@lca.pw> <20191007143002.l37bt2lzqtnqjqxu@pathway.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191007143002.l37bt2lzqtnqjqxu@pathway.suse.cz> 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 [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: 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; sclp_init_state = sclp_init_state_initializing; - sclp_read_sccb = (void *) __get_free_page(GFP_ATOMIC | GFP_DMA); - sclp_init_sccb = (void *) __get_free_page(GFP_ATOMIC | GFP_DMA); BUG_ON(!sclp_read_sccb || !sclp_init_sccb); /* Set up variables */ INIT_LIST_HEAD(&sclp_req_queue); -- Michal Hocko SUSE Labs