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=-3.3 required=3.0 tests=BAYES_00, BUG6152_INVALID_DATE_TZ_ABSURD,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU, HEADER_FROM_DIFFERENT_DOMAINS,INVALID_DATE_TZ_ABSURD,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no 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 95210C4346E for ; Thu, 24 Sep 2020 09:21:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 36CD4212CC for ; Thu, 24 Sep 2020 09:21:55 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="qlx733Zr"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="Hw8BadpY" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727330AbgIXJVy (ORCPT ); Thu, 24 Sep 2020 05:21:54 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38514 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726710AbgIXJVx (ORCPT ); Thu, 24 Sep 2020 05:21:53 -0400 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BDE96C0613CE for ; Thu, 24 Sep 2020 02:21:53 -0700 (PDT) From: John Ogness DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1600939312; 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=fMAvwwFV30HK4hEY4FeabCBTMHVTzPS6tY9DUMcJlTM=; b=qlx733Zr8+YqmCkWvzeX3ZdiQudovl7ZSFdTEMjGtEr4OXU7CvxQmmZ+cRNoO9Q8o30YmQ IfJ35w6W4xBqS2Fyhq0XtU2drHWkRdrC85eOl5OuwIShY/33mmt4eKZNvmUhsuJs/cVrX+ jsbRPyeQsEdpR0AQRnvCvrflwVUKvIYf9HfySilYnD0rXY9rVZKCXb4TzaB6nWv5gxhxvm PRxWL67G/B8/qYzGFxnbemX+ViJnXIgv8hdH/n4APBcOiBzk5xgJZQuK4Va9sUr0tzKZGL yxKrwHkvYDqb66QcyfG3YpLXhKxhVneeuizu9i0Ssu7hFOvBuNgge7ITNJa6Pw== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1600939312; 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=fMAvwwFV30HK4hEY4FeabCBTMHVTzPS6tY9DUMcJlTM=; b=Hw8BadpY62hy9A5xiGbX1YcvD/LmaDrFsBshXnmEh83j4JbSIcZAH4/5/+N+6DJYALAkQ5 g3bym75Y5gX4FeDQ== To: Sergey Senozhatsky Cc: Petr Mladek , Sergey Senozhatsky , Sergey Senozhatsky , Steven Rostedt , Linus Torvalds , Greg Kroah-Hartman , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: Re: [PATCH printk 4/5] printk: remove logbuf_lock, add syslog_lock In-Reply-To: <20200924084541.GA541@jagdpanzerIV.localdomain> References: <20200922153816.5883-1-john.ogness@linutronix.de> <20200922153816.5883-5-john.ogness@linutronix.de> <20200924084541.GA541@jagdpanzerIV.localdomain> Date: Thu, 24 Sep 2020 11:27:51 +0206 Message-ID: <87d02b7cnk.fsf@jogness.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020-09-24, Sergey Senozhatsky wrote: > [..] >> void printk_safe_flush_on_panic(void) >> { >> - /* >> - * Make sure that we could access the main ring buffer. >> - * Do not risk a double release when more CPUs are up. >> - */ >> - if (raw_spin_is_locked(&logbuf_lock)) { >> - if (num_online_cpus() > 1) >> - return; >> - >> - debug_locks_off(); >> - raw_spin_lock_init(&logbuf_lock); > > If we panic() with syslog_lock locked then we need to zap syslog_lock > and disable debug_locks for kmsg_dump() to succeed. Otherwise panic() > may deadlock: > > syslog_lock_irq() > -> NMI > panic() > ... > printk_safe_flush_on_panic(); > kmsg_dump(KMSG_DUMP_PANIC); // syslog_lock_irqsave(flags) > ... > > Did I miss the point where we make sure that kmsg_dump() will not > deadlock? You are correct. Thanks. @clear_seq should either become atomic64 or kmsg_dump() should start from the oldest available message (seq=0). John Ogness