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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 43D3CEB8FD2 for ; Wed, 6 Sep 2023 13:58:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234630AbjIFN6C (ORCPT ); Wed, 6 Sep 2023 09:58:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59284 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229907AbjIFN55 (ORCPT ); Wed, 6 Sep 2023 09:57:57 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [IPv6:2001:67c:2178:6::1c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id ACE5710F5 for ; Wed, 6 Sep 2023 06:57:49 -0700 (PDT) Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id 6D3BD22302; Wed, 6 Sep 2023 13:57:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1694008668; 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=q7OA73ODeuzanjyJ7EdEc/ybpFb3Wkpfax58iwx/ejk=; b=JLreQiKBrnJ/262TLjvojvwGvo+cWAQJRs44luEzvaAIzVPfuA1+463ffos5L9Mxzoqe2V 42gufn4LdhH526Cl18Dq8jGQEZtYK4cIlRZdn/bQJCI3E5JoMl6oBMdAUeufe3/STv381+ D7aBgwOUWISJ07jSjegKjASgCwBZTFk= Received: from suse.cz (pmladek.tcp.ovpn2.prg.suse.de [10.100.208.146]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id 1E7D22C142; Wed, 6 Sep 2023 13:57:48 +0000 (UTC) Date: Wed, 6 Sep 2023 15:57:47 +0200 From: Petr Mladek To: John Ogness Cc: Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: Re: [PATCH printk v3 4/7] printk: nbcon: Add ownership state functions Message-ID: References: <20230903150539.245076-1-john.ogness@linutronix.de> <20230903150539.245076-5-john.ogness@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230903150539.245076-5-john.ogness@linutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun 2023-09-03 17:11:36, John Ogness wrote: > From: Thomas Gleixner > > Provide functions that are related to the safe handover mechanism > and allow console drivers to dynamically specify unsafe regions: > > - nbcon_context_can_proceed() > > Invoked by a console owner to check whether a handover request > is pending or whether the console has been taken over by another > context. If a handover request is pending, this function will > also perform the handover, thus cancelling its own ownership. > > - nbcon_context_update_unsafe() > > Invoked by a console owner to denote that the driver is about > to enter or leave a critical region where a take over is unsafe. > This function is also a cancellation point where loss of > ownership can occur. > > The unsafe state is stored in the console state and allows a > new context to make informed decisions whether to attempt a > takeover of such a console. The unsafe state is also available > to the driver so that it can make informed decisions about the > required actions and possibly take a special emergency path. > > Co-developed-by: John Ogness > Signed-off-by: John Ogness > Signed-off-by: Thomas Gleixner (Intel) Reviewed-by: Petr Mladek See one comment below. > --- a/kernel/printk/nbcon.c > +++ b/kernel/printk/nbcon.c > @@ -518,6 +517,112 @@ static void nbcon_context_release(struct nbcon_context *ctxt) > ctxt->pbufs = NULL; > } > > +/** > + * nbcon_context_can_proceed - Check whether ownership can proceed > + * @ctxt: The nbcon context from nbcon_context_try_acquire() > + * @cur: The current console state > + * > + * Return: True if this context still owns the console. False if > + * ownership was handed over or taken. > + * > + * Must be invoked after the record was dumped into the assigned buffer > + * and at appropriate safe places in the driver. > + * > + * When this function returns false then the calling context no longer owns > + * the console and is no longer allowed to go forward. In this case it must > + * back out immediately and carefully. The buffer content is also no longer > + * trusted since it no longer belongs to the calling context. > + */ > +static bool nbcon_context_can_proceed(struct nbcon_context *ctxt, struct nbcon_state *cur) > +{ > + unsigned int cpu = smp_processor_id(); > + > + /* Make sure this context still owns the console. */ > + if (!nbcon_owner_matches(cur, cpu, ctxt->prio)) > + return false; > + > + /* The console owner can proceed if there is no waiter. */ > + if (cur->req_prio == NBCON_PRIO_NONE) > + return true; > + > + /* > + * A console owner within an unsafe region is always allowed to > + * proceed, even if there are waiters. It can perform a handover > + * when exiting the unsafe region. Otherwise the waiter will > + * need to perform an unsafe hostile takeover. > + */ > + if (cur->unsafe) > + return true; > + > + /* Waiters always have higher priorities than owners. */ > + WARN_ON_ONCE(cur->req_prio <= cur->prio); > + > + /* > + * Having a safe point for take over and eventually a few > + * duplicated characters or a full line is way better than a > + * hostile takeover. Post processing can take care of the garbage. > + * Release and hand over. > + */ > + nbcon_context_release(ctxt); > + > + /* > + * It is not known whether the handover succeeded. The outermost It was not immediately clear to me what exactly "handover succeeded" did mean. I would write: * It is not clear whether the waiter really took the lock * and re-printed the record. The outermost calsite... > + * callsite has to make the final decision whether printing > + * should proceed or not (via reacquire, possibly hostile). The > + * console is now unlocked so go back all the way instead of > + * trying to implement heuristics in tons of places. > + */ > + return false; > +} Best Regards, Petr