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 7D3E4EB64D7 for ; Mon, 26 Jun 2023 13:15:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230498AbjFZNPD (ORCPT ); Mon, 26 Jun 2023 09:15:03 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59556 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229904AbjFZNOo (ORCPT ); Mon, 26 Jun 2023 09:14:44 -0400 Received: from outbound-smtp46.blacknight.com (outbound-smtp46.blacknight.com [46.22.136.58]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 10DBD10A for ; Mon, 26 Jun 2023 06:14:43 -0700 (PDT) Received: from mail.blacknight.com (pemlinmail02.blacknight.ie [81.17.254.11]) by outbound-smtp46.blacknight.com (Postfix) with ESMTPS id 43A69FABDD for ; Mon, 26 Jun 2023 14:14:41 +0100 (IST) Received: (qmail 26953 invoked from network); 26 Jun 2023 13:14:41 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.21.103]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 26 Jun 2023 13:14:40 -0000 Date: Mon, 26 Jun 2023 14:14:38 +0100 From: Mel Gorman To: Sebastian Andrzej Siewior Cc: Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Luis Claudio R. Goncalves" , Andrew Morton , Boqun Feng , Ingo Molnar , John Ogness , Peter Zijlstra , Petr Mladek , Tetsuo Handa , Thomas Gleixner , Waiman Long , Will Deacon Subject: Re: [PATCH v3 2/2] mm/page_alloc: Use write_seqlock_irqsave() instead write_seqlock() + local_irq_save(). Message-ID: <20230626131438.54bupbuujbqe4b5a@techsingularity.net> References: <20230623171232.892937-1-bigeasy@linutronix.de> <20230623171232.892937-3-bigeasy@linutronix.de> <20230623201517.yw286Knb@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20230623201517.yw286Knb@linutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jun 23, 2023 at 10:15:17PM +0200, Sebastian Andrzej Siewior wrote: > __build_all_zonelists() acquires zonelist_update_seq by first disabling > interrupts via local_irq_save() and then acquiring the seqlock with > write_seqlock(). This is troublesome and leads to problems on > PREEMPT_RT. The problem is that the inner spinlock_t becomes a sleeping > lock on PREEMPT_RT and must not be acquired with disabled interrupts. > > The API provides write_seqlock_irqsave() which does the right thing in > one step. > printk_deferred_enter() has to be invoked in non-migrate-able context to > ensure that deferred printing is enabled and disabled on the same CPU. > This is the case after zonelist_update_seq has been acquired. > > There was discussion on the first submission that the order should be: > local_irq_disable(); > printk_deferred_enter(); > write_seqlock(); > > to avoid pitfalls like having an unaccounted printk() coming from > write_seqlock_irqsave() before printk_deferred_enter() is invoked. The > only origin of such a printk() can be a lockdep splat because the > lockdep annotation happens after the sequence count is incremented. > This is exceptional and subject to change. > > It was also pointed that PREEMPT_RT can be affected by the printk > problem since its write_seqlock_irqsave() does not really disable > interrupts. This isn't the case because PREEMPT_RT's printk > implementation differs from the mainline implementation in two important > aspects: > - Printing happens in a dedicated threads and not at during the > invocation of printk(). > - In emergency cases where synchronous printing is used, a different > driver is used which does not use tty_port::lock. > > Acquire zonelist_update_seq with write_seqlock_irqsave() and then defer > printk output. > > Fixes: 1007843a91909 ("mm/page_alloc: fix potential deadlock on zonelist_update_seq seqlock") > Signed-off-by: Sebastian Andrzej Siewior > Acked-by: Michal Hocko Acked-by: Mel Gorman -- Mel Gorman SUSE Labs