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=-5.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 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 A668EC433DF for ; Tue, 11 Aug 2020 16:19:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7EEF42076C for ; Tue, 11 Aug 2020 16:19:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1597162773; bh=eSlY7bnaywQP2Qzm8/RoLCGGkJGUHuYdHvojM9qWpDg=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=hRH43xRWeVkS9qmf2+7yWIQ2O0c7r3HYTms17tHhtPjapu4J3ToLcy3WXvA4wSv+6 8xyNe2PzqMRm9pNIb0v+aRJhRTad0YYJKzvWsbUsY3FFuXtmb7s3oz4+kb92hXtLKp ddhtm9Y4d7JwmQ2NBFV3mYBWJ0ozTj1xxi2gFGBY= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729078AbgHKQTd (ORCPT ); Tue, 11 Aug 2020 12:19:33 -0400 Received: from mail.kernel.org ([198.145.29.99]:37780 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728797AbgHKQTc (ORCPT ); Tue, 11 Aug 2020 12:19:32 -0400 Received: from paulmck-ThinkPad-P72.home (unknown [50.45.173.55]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 2AA7B206B5; Tue, 11 Aug 2020 16:19:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1597162772; bh=eSlY7bnaywQP2Qzm8/RoLCGGkJGUHuYdHvojM9qWpDg=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=ZVEGIHWMLJLCWOXy2zPXwFEWPqlLABJ4U8TkB9Q1qgj3XY+i2S7NSiY6Xz//PNRaI W1z5kGe5Zl+SR/6OahDUitkov36/DQjBJQXoIeN50kZLyYc8HbgSx4AggrRCiGUfct CMx658Og1vNxfKwgzGEWEAKgDLMRiivDZ3326qXY= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id D559B352308E; Tue, 11 Aug 2020 09:19:31 -0700 (PDT) Date: Tue, 11 Aug 2020 09:19:31 -0700 From: "Paul E. McKenney" To: Thomas Gleixner Cc: Michal Hocko , Uladzislau Rezki , LKML , RCU , linux-mm@kvack.org, Andrew Morton , Vlastimil Babka , Matthew Wilcox , "Theodore Y . Ts'o" , Joel Fernandes , Sebastian Andrzej Siewior , Oleksiy Avramchenko Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag Message-ID: <20200811161931.GA1746@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200809204354.20137-1-urezki@gmail.com> <20200809204354.20137-2-urezki@gmail.com> <20200810123141.GF4773@dhcp22.suse.cz> <20200810160739.GA29884@pc636> <20200810192525.GG4773@dhcp22.suse.cz> <87pn7x6y4a.fsf@nanos.tec.linutronix.de> <20200811153327.GW4295@paulmck-ThinkPad-P72> <87h7t96ve3.fsf@nanos.tec.linutronix.de> <20200811160240.GX4295@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200811160240.GX4295@paulmck-ThinkPad-P72> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Tue, Aug 11, 2020 at 09:02:40AM -0700, Paul E. McKenney wrote: > On Tue, Aug 11, 2020 at 05:43:16PM +0200, Thomas Gleixner wrote: > > "Paul E. McKenney" writes: > > > On Tue, Aug 11, 2020 at 04:44:21PM +0200, Thomas Gleixner wrote: > > >> Now RCU creates a new thing which enforces to make page allocation in > > >> atomic context possible on RT. What for? > > >> > > >> What's the actual use case in truly atomic context for this new thing on > > >> an RT kernel? > > > > > > It is not just RT kernels. CONFIG_PROVE_RAW_LOCK_NESTING=y propagates > > > this constraint to all configurations, and a patch in your new favorite > > > subsystem really did trigger this lockdep check in a non-RT kernel. > > > > > >> The actual RCU code disabling interrupts is an implementation detail > > >> which can easily be mitigated with a local lock. > > > > > > In this case, we are in raw-spinlock context on entry to kfree_rcu(). > > > > Where? > > Some BPF code that needs to process and free a list. As noted above, > this is a patch rather than something that is already in mainline. > Not surprising, though, given call_rcu() invocations in similar contexts. > > Yes, we can perhaps rework all current and future callers to avoid > invoking both call_rcu() and kfree_rcu() from raw atomic context, but > the required change to permit this is quite a bit simpler. I should hasten to add that from what I can see right now, the required change allows telling the memory allocator bail out instead of acquiring a non-raw spinlock. I am absolutely not advocating converting the allocator's spinlocks to raw spinlocks. Thanx, Paul