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=-15.2 required=3.0 tests=BAYES_00, BUG6152_INVALID_DATE_TZ_ABSURD,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, INVALID_DATE_TZ_ABSURD,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 D7C7CC433E0 for ; Mon, 1 Feb 2021 16:52:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8D6DE64ECC for ; Mon, 1 Feb 2021 16:52:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231757AbhBAQw3 (ORCPT ); Mon, 1 Feb 2021 11:52:29 -0500 Received: from Galois.linutronix.de ([193.142.43.55]:58678 "EHLO galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229849AbhBAQv7 (ORCPT ); Mon, 1 Feb 2021 11:51:59 -0500 From: John Ogness DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1612198276; h=from:from:reply-to:subject:subject: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=M0U85P18JGolozwquk8qN9IwcXoRxo63HyyVjJHZEc8=; b=KYw75ikcv2s0KEOGlQaMjbz3g2dr68+Vqc4WQszJi0x3GKMoq1j8NP9GpES9apaQX0ITBz O2HeMQ6Uyskw+a2IGpPBOHuPDQ4iA6nQaAVGbRQPF8Co2h+XgT73rpTA7orq9haap9xMUh QROqbUCaj6ZKv3JuNvcsbEQjYZAJN+HHGaLq4XdlcXmWSK5ek67XkJRlU0y9EuWBc5++1E tu/2445aqo0CtxKCX9VSZCoHqNRqJ7xRMyYUpQ6cE3ucfV3d3jgCx2GnrHLDQgh+lSwNDO V6uiJ5+XpyfeVukclrnMYGqhhGrbhWReRuAFWmCppPYHY3tlHueDBwzw9y9mjA== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1612198276; h=from:from:reply-to:subject:subject: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=M0U85P18JGolozwquk8qN9IwcXoRxo63HyyVjJHZEc8=; b=kq8UO9DfsPpGaw/6A+z9zNFGr9cO16/wJ/venR82ZZ2aCYCvJXYMZ3Bbrx63pmJE5ox41K SjCN1f7QSLRUeLDg== To: Petr Mladek 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 In-Reply-To: References: <20210126211551.26536-1-john.ogness@linutronix.de> <20210126211551.26536-10-john.ogness@linutronix.de> Date: Mon, 01 Feb 2021 17:57:14 +0106 Message-ID: <875z3bk9z1.fsf@jogness.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org (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. John Ogness