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=-13.7 required=3.0 tests=BAYES_00, 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 60669C433DB for ; Mon, 1 Feb 2021 16:55:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EE20F64EA1 for ; Mon, 1 Feb 2021 16:55:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232157AbhBAQzz convert rfc822-to-8bit (ORCPT ); Mon, 1 Feb 2021 11:55:55 -0500 Received: from lithops.sigma-star.at ([195.201.40.130]:40258 "EHLO lithops.sigma-star.at" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232128AbhBAQz1 (ORCPT ); Mon, 1 Feb 2021 11:55:27 -0500 Received: from localhost (localhost [127.0.0.1]) by lithops.sigma-star.at (Postfix) with ESMTP id A14DF608A38A; Mon, 1 Feb 2021 17:54:42 +0100 (CET) Received: from lithops.sigma-star.at ([127.0.0.1]) by localhost (lithops.sigma-star.at [127.0.0.1]) (amavisd-new, port 10032) with ESMTP id QdTC7UL6ZzsC; Mon, 1 Feb 2021 17:54:41 +0100 (CET) Received: from localhost (localhost [127.0.0.1]) by lithops.sigma-star.at (Postfix) with ESMTP id 421C5608A38F; Mon, 1 Feb 2021 17:54:41 +0100 (CET) Received: from lithops.sigma-star.at ([127.0.0.1]) by localhost (lithops.sigma-star.at [127.0.0.1]) (amavisd-new, port 10026) with ESMTP id qD71sWIEK9ZZ; Mon, 1 Feb 2021 17:54:41 +0100 (CET) Received: from lithops.sigma-star.at (lithops.sigma-star.at [195.201.40.130]) by lithops.sigma-star.at (Postfix) with ESMTP id 1EFC1608A38A; Mon, 1 Feb 2021 17:54:41 +0100 (CET) Date: Mon, 1 Feb 2021 17:54:41 +0100 (CET) From: Richard Weinberger To: John Ogness Cc: Petr Mladek , Sergey Senozhatsky , Sergey Senozhatsky , Steven Rostedt , tglx , linux-kernel , Thomas Meyer Message-ID: <1829482025.350347.1612198481059.JavaMail.zimbra@nod.at> In-Reply-To: <875z3bk9z1.fsf@jogness.linutronix.de> References: <20210126211551.26536-1-john.ogness@linutronix.de> <20210126211551.26536-10-john.ogness@linutronix.de> <875z3bk9z1.fsf@jogness.linutronix.de> Subject: Re: [PATCH printk-rework 09/12] um: synchronize kmsg_dumper MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT X-Originating-IP: [195.201.40.130] X-Mailer: Zimbra 8.8.12_GA_3807 (ZimbraWebClient - FF78 (Linux)/8.8.12_GA_3809) Thread-Topic: synchronize kmsg_dumper Thread-Index: rGdK+jOn3dkfStTjhSYaERhsgfIOaw== Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org John, ----- Ursprüngliche Mail ----- > Von: "John Ogness" > An: "Petr Mladek" > CC: "Sergey Senozhatsky" , "Sergey Senozhatsky" , > "Steven Rostedt" , "tglx" , "linux-kernel" , > "Thomas Meyer" , "richard" > Gesendet: Montag, 1. Februar 2021 17:51:14 > Betreff: Re: [PATCH printk-rework 09/12] um: synchronize kmsg_dumper > (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. > >> IMHO, this patch is not needed. > > I am not familiar enough with ARCH=um to know if dumps can be triggered > on multiple CPUs simultaneously. Perhaps ThomasM or Richard can chime in > here. Well, uml has no SMP support, so no parallel dumps. :-) Thanks, //richard