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=-5.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 DCA88C4332E for ; Tue, 2 Mar 2021 15:09:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9EF6B6186A for ; Tue, 2 Mar 2021 15:09:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1448555AbhCBPFk (ORCPT ); Tue, 2 Mar 2021 10:05:40 -0500 Received: from mx2.suse.de ([195.135.220.15]:42022 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1350774AbhCBMts (ORCPT ); Tue, 2 Mar 2021 07:49:48 -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=1614689304; 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=LmimWd63WkUfPWADlD3zBnnPZ6h7AIB2oXCZzzXjlJw=; b=mPQMTl8fVDM8qIPI5TkL3p1CaW2GOGMEx9OkvNmBms+ceLRjwgbXn3z390vKN806puhD2k cPbmjYXeCCaGE9ANqE0n2RVQRigw7DJw/ZEIEEfKc7+D/QJoydsSU0laPEz2jkqbC1h/5V vNprbyefwf+gWEYUGsnXZPAtZlcKSbg= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id DFAE3AF63; Tue, 2 Mar 2021 12:48:23 +0000 (UTC) Date: Tue, 2 Mar 2021 13:48:23 +0100 From: Petr Mladek To: John Ogness Cc: Sergey Senozhatsky , Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org, Miquel Raynal , Richard Weinberger , Vignesh Raghavendra , linux-mtd@lists.infradead.org Subject: Re: [PATCH next v3 02/15] mtd: mtdoops: synchronize kmsg_dumper Message-ID: References: <20210225202438.28985-1-john.ogness@linutronix.de> <20210225202438.28985-3-john.ogness@linutronix.de> <87tuptq1fc.fsf@jogness.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87tuptq1fc.fsf@jogness.linutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 2021-03-02 11:45:27, John Ogness wrote: > On 2021-03-01, Petr Mladek wrote: > >> The kmsg_dumper can be called from any context and CPU, possibly > >> from multiple CPUs simultaneously. Since the writing of the buffer > >> can occur from a later scheduled work queue, the oops buffer must > >> be protected against simultaneous dumping. > >> > >> Use an atomic bit to mark when the buffer is protected. Release the > >> protection in between setting the buffer and the actual writing in > >> order for a possible panic (immediate write) to be written during > >> the scheduling of a previous oops (delayed write). > > > > Just to be sure. You did not use spin lock to prevent problems > > with eventual double unlock in panic(). Do I get it correctly, > > please? > > I do not understand what possible double unlock you are referring to. I was wrong. I meant the tricks that are under in console drivrers, for example: static void mvebu_uart_console_write(struct console *co, const char *s, unsigned int count) { int locked = 1; if (oops_in_progress) locked = spin_trylock_irqsave(&port->lock, flags); else spin_lock_irqsave(&port->lock, flags); /* do the job */ if (locked) spin_unlock_irqrestore(&port->lock, flags); } But this is not a problem here because the kmsg dumper bails out when the lock could not be taken. > I chose not to use spinlocks because I wanted something that does not > cause any scheduling or preemption side-effects for mtd. The mtd dumper > sometimes dumps directly, sometimes delayed (via scheduled work), and > they use different mtd callbacks in different contexts. > > mtd_write() expects to be called in a non-atomic context. The callbacks > can take a mutex. Makes sense. Could you please mention this in the commit message? Best Regards, Petr