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=-15.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 2E1F9C433E0 for ; Tue, 2 Feb 2021 13:27:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E046D64EC5 for ; Tue, 2 Feb 2021 13:27:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231858AbhBBN06 (ORCPT ); Tue, 2 Feb 2021 08:26:58 -0500 Received: from mx2.suse.de ([195.135.220.15]:34252 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231180AbhBBN0y (ORCPT ); Tue, 2 Feb 2021 08:26:54 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1612272367; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=0UoTc8bOW9a73YYpJw8OzZ66a6bNPlPn/jdMLo9zjDg=; b=YGJauBWO8UBxLjDoL61JsDvobUm18oKNkFq2qK6MuEI7cAZilY607byVPOkhD6IQGqnzke ZZzlh+OaqfvRktTV+jbpn+Qhv0u46spOZJS1mk0IPfSVm8gr+Ay/pld/I6GIK2lPzVaa/7 Tsd8HZqIQaSzdkJFRbD0mM6nUrRC63o= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 876D7AF9A; Tue, 2 Feb 2021 13:26:07 +0000 (UTC) Date: Tue, 2 Feb 2021 14:26:06 +0100 From: Petr Mladek To: John Ogness Cc: Sergey Senozhatsky , Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org, Thomas Meyer , Richard Weinberger Subject: Re: [PATCH printk-rework 09/12] um: synchronize kmsg_dumper Message-ID: References: <20210126211551.26536-1-john.ogness@linutronix.de> <20210126211551.26536-10-john.ogness@linutronix.de> <875z3bk9z1.fsf@jogness.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <875z3bk9z1.fsf@jogness.linutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 2021-02-01 17:57:14, John Ogness wrote: > (Added CC: Thomas Meyer, Richard Weinberger) > > On 2021-02-01, Petr Mladek wrote: > >> In preparation for removing printk's @logbuf_lock, dumpers that have > >> assumed to be protected against parallel calls must provide their own > >> synchronization. Add a locally static spinlock to synchronize the > >> kmsg_dump call and temporary buffer usage. > >> > >> Signed-off-by: John Ogness > >> --- > >> arch/um/kernel/kmsg_dump.c | 8 ++++++++ > >> 1 file changed, 8 insertions(+) > >> > >> diff --git a/arch/um/kernel/kmsg_dump.c b/arch/um/kernel/kmsg_dump.c > >> index f38349ad00ea..173999422ed8 100644 > >> --- a/arch/um/kernel/kmsg_dump.c > >> +++ b/arch/um/kernel/kmsg_dump.c > >> @@ -1,5 +1,6 @@ > >> // SPDX-License-Identifier: GPL-2.0 > >> #include > >> +#include > >> #include > >> #include > >> #include > >> @@ -9,8 +10,10 @@ static void kmsg_dumper_stdout(struct kmsg_dumper *dumper, > >> enum kmsg_dump_reason reason, > >> struct kmsg_dumper_iter *iter) > >> { > >> + static DEFINE_SPINLOCK(lock); > >> static char line[1024]; > >> struct console *con; > >> + unsigned long flags; > >> size_t len = 0; > >> > >> /* only dump kmsg when no console is available */ > >> @@ -25,11 +28,16 @@ static void kmsg_dumper_stdout(struct kmsg_dumper *dumper, > >> if (con) > >> return; > >> > >> + if (!spin_trylock_irqsave(&lock, flags)) > >> + return; > >> + > >> printf("kmsg_dump:\n"); > >> while (kmsg_dump_get_line(iter, true, line, sizeof(line), &len)) { > >> line[len] = '\0'; > >> printf("%s", line); > >> } > >> + > >> + spin_unlock_irqrestore(&lock, flags); > > > > What exactly is synchronized here, please? > > Access to @line buffer or @iter or both? > > @line is being synchronized. @iter does not require synchronization. > > > It looks to me that the access to @line buffer was not synchronized > > before. kmsg_dump_get_line() used a lock internally but > > it was not synchronized with the later printf("%s", line); > > The line was previously synchronized for the kmsg_dump_get_line() > call. But yes, it was not synchronized after the call, which is a bug if > the dump is triggered on multiple CPUs simultaneously. The commit > message should also mention that it is handling that bug. Yes, please update the commit message. In fact, I think that mentioning logbuf_lock might is a bit misleading. This patch fixes an old bug. It was found when auditing code that is using the kmsg_dump_get_line() and might be affected the logbuf_lock removal. Feel free to use: Reviewed-by: Petr Mladek Finally thanks Richard for the input about SMP on UM arch. Best Regards, Petr