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 17348C6FD18 for ; Wed, 19 Apr 2023 12:14:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233187AbjDSMOW (ORCPT ); Wed, 19 Apr 2023 08:14:22 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35554 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233074AbjDSMOU (ORCPT ); Wed, 19 Apr 2023 08:14:20 -0400 Received: from smtp-out2.suse.de (smtp-out2.suse.de [195.135.220.29]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0064755BE for ; Wed, 19 Apr 2023 05:14:17 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out2.suse.de (Postfix) with ESMTPS id 6E6C81FD8B; Wed, 19 Apr 2023 12:14:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1681906456; 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=P75V+PHCMLRraJ75ssd9S6FUz/in7e36rjP7Uvyk1M0=; b=j1vKSRL4ILQL3REI4O2rzaEGsUxK5VAzhbkZMe3cXH2+EiMZ4JD0to7agaVvNPf4xX1GTt nWeh3DqzBRugu0RerHtiSW3a+mPmyaeEKf9k5EucFhUfLp93K5jdYJbBpTIUiga3yt9D6X 0o3/vQsH2C7yJkgzXUE3PSX68xpt7bA= Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 5283C13580; Wed, 19 Apr 2023 12:14:16 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id 86TOERjbP2S8fwAAMHmgww (envelope-from ); Wed, 19 Apr 2023 12:14:16 +0000 Date: Wed, 19 Apr 2023 14:14:15 +0200 From: Michal Hocko To: Petr Mladek Cc: John Ogness , Sergey Senozhatsky , Steven Rostedt , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] printk: Enough to disable preemption in printk deferred context Message-ID: References: <20230419074210.17646-1-pmladek@suse.com> <87r0sg5jin.fsf@jogness.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 19-04-23 14:03:29, Petr Mladek wrote: > On Wed 2023-04-19 11:11:52, John Ogness wrote: > > On 2023-04-19, Petr Mladek wrote: > > > it is safe to interrupt one writer now. The preemption still > > > has to be disabled because the deferred context is CPU specific. > > > > Really it is enough to disable migration. > > True. But it gets too far to my taste. As you describe below. > It affects all printk's on the CPU. > > Sigh, even the enabled intrrupts might be questionable. For example, > when the iterrupt is from a watchdog and want's to report a stall. > > > We need to keep an eye on the usage of this function. By allowing > > interrupts and preemption, it means that other printk's on that CPU will > > also be deferred if the context interrupted within the deferred block. > > A solution would be to make this more clear in the comment. > Something like: > > /* > * The printk_deferred_enter/exit macros are available only as a hack. > * They define a per-CPU context where all printk console printing is > * deferred because it might cause a deadlock otherwise. > * > * The API user is responsible for calling the corresponding enter/exit > * pair on the same CPU. It is highly recommended to use them only in > * a context with interrupts disabled. Otherwise, other unrelated > * printk() calls might be deferred when they interrupt/preempt > * the deferred code section. > */ This looks better but I would argue that as a potential user of those I would appreciate less internal implementation details and more instructions on how/when to use it. What about something like this? /* * The printk_deferred_enter/exit macros are available only as a hack * for code paths which are prone to printk related deadlocks. That * might be caused by locking context around printk which can be reused * directly or indirectly by lower level printk infrastructure. * * Any new use of these MUST be consulted with printk maintainers as the * use might have some unexpected side effects on the printk * infrastructure. * * enter/exit pair must be called from the same CPU without any * preemption in between. */ -- Michal Hocko SUSE Labs