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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 5F17DC282C2 for ; Wed, 13 Feb 2019 14:15:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 37DCE222B5 for ; Wed, 13 Feb 2019 14:15:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732842AbfBMOP5 (ORCPT ); Wed, 13 Feb 2019 09:15:57 -0500 Received: from Galois.linutronix.de ([146.0.238.70]:46615 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726654AbfBMOP4 (ORCPT ); Wed, 13 Feb 2019 09:15:56 -0500 Received: from localhost ([127.0.0.1] helo=vostro.local) by Galois.linutronix.de with esmtp (Exim 4.80) (envelope-from ) id 1gtvKC-000314-OO; Wed, 13 Feb 2019 15:15:44 +0100 From: John Ogness To: Sergey Senozhatsky Cc: linux-kernel@vger.kernel.org, Peter Zijlstra , Petr Mladek , Steven Rostedt , Daniel Wang , Andrew Morton , Linus Torvalds , Greg Kroah-Hartman , Alan Cox , Jiri Slaby , Peter Feiner , linux-serial@vger.kernel.org, Sergey Senozhatsky Subject: Re: [RFC PATCH v1 00/25] printk: new implementation References: <20190212143003.48446-1-john.ogness@linutronix.de> <20190213014141.GB8097@jagdpanzerIV> Date: Wed, 13 Feb 2019 15:15:42 +0100 In-Reply-To: <20190213014141.GB8097@jagdpanzerIV> (Sergey Senozhatsky's message of "Wed, 13 Feb 2019 10:41:41 +0900") Message-ID: <878syj22r5.fsf@linutronix.de> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.4 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2019-02-13, Sergey Senozhatsky wrote: >> - console_flush_on_panic() currently is a NOP. It is pretty clear how >> this could be implemented if atomic_write was available. But if no >> such console is registered, it is not clear what should be done. Is >> this function really even needed? > > If you now rely on a fully preemptible printk kthread to flush > pending logbuf messages, then console_flush_on_panic() is your > only chance to see those pending logbuf messages on the serial > console when the system dies. Anything critical would have already been immediately print to the emergency consoles. And if an emergency console was available, console_flush_on_panic() could be a special case where _all_ unseen messages (regardless of importance) are printed to the emergency console. > Non-atomic consoles should become atomic once you call bust_spinlocks(1), > this is what we currently have: > > panic() > bust_spinlocks(1) // sets oops_in_progress > console_flush_on_panic() > call_console_drivers() > -> serial_driver_write() > if (oops_in_progress) > locked = spin_trylock_irqsave(&port->lock); > uart_console_write(); > if (locked) > spin_unlock_irqrestore(&port->lock); I don't like bust_spinlocks() because drivers end up implementing oops_in_progress with exactly that... ignoring their own locks. I prefer consoles are provided with a locking mechanism that they can use to support a separate NMI-safe write function. My series introduces console_atomic_lock() for exactly this purpose. But this doesn't help here. Here we are talking about a crashing system that does _not_ have an emergency console. And in this case I would say messages would be lost (just like they are now if all you have is a vt console and it was busy). I suppose we could keep the current bust_spinlocks() stuff for the special case that there are no emergency consoles available. It's better than nothing, but also not really reliable. Preferrably we figure out how to implement write_atomic for all console drivers. John Ogness