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 63998C433E0 for ; Thu, 16 Jul 2020 15:20:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4347C20874 for ; Thu, 16 Jul 2020 15:20:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1594912833; bh=PPJ9m8c9qSJB+p9W3coV52UIGzOiNECbZW1ePPsr2qM=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=OEm7CTIccOoiUjqiVKleweKGUbUCr9JbiVWIQYVLg5YcBM54hCJWDf2rVtTm+46+E vPJEla5nxhKbM7MGL+2P3FHlWcvjOxJgeT4lhUSdC0RPd2PD3hu5Bvdac4C7YrukSO F0pl6j7RiVDUEgGazqyWHMgCjLNF6NUxIbPyf2Hg= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728908AbgGPPU3 (ORCPT ); Thu, 16 Jul 2020 11:20:29 -0400 Received: from mail.kernel.org ([198.145.29.99]:42506 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729123AbgGPPU1 (ORCPT ); Thu, 16 Jul 2020 11:20:27 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-111-31.bvtn.or.frontiernet.net [50.39.111.31]) (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 2DC4520825; Thu, 16 Jul 2020 15:20:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1594912827; bh=PPJ9m8c9qSJB+p9W3coV52UIGzOiNECbZW1ePPsr2qM=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=IJkS2fSEK6rAbF2nOQXWQ+mlBhg91YptiouLkxoGU2kMdnrFnJEtP6mwTKvWJ5p93 h2jTbQoHNU63OnRgqnNKBPGBmPTZiNB/6qYUEUGx3Eehf5hNky1gGDduO0hIB3gjWz ruJegvsRwZeroGPbM7kUVtYwOTYHlK13iETsA++A= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 0EA4A3522635; Thu, 16 Jul 2020 08:20:27 -0700 (PDT) Date: Thu, 16 Jul 2020 08:20:27 -0700 From: "Paul E. McKenney" To: Sebastian Andrzej Siewior Cc: Uladzislau Rezki , LKML , RCU , linux-mm@kvack.org, Andrew Morton , "Theodore Y . Ts'o" , Matthew Wilcox , Joel Fernandes , Oleksiy Avramchenko Subject: Re: [PATCH 1/1] rcu/tree: Drop the lock before entering to page allocator Message-ID: <20200716152027.GQ9247@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200715183537.4010-1-urezki@gmail.com> <20200715185628.7b4k3o5efp4gnbla@linutronix.de> <20200715190243.GA26735@pc636> <20200715193250.axntj7jdt6bw52dr@linutronix.de> <20200715221449.GJ9247@paulmck-ThinkPad-P72> <20200716141421.fzwf4tedr6rixd6d@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200716141421.fzwf4tedr6rixd6d@linutronix.de> 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 Thu, Jul 16, 2020 at 04:14:21PM +0200, Sebastian Andrzej Siewior wrote: > On 2020-07-15 15:14:49 [-0700], Paul E. McKenney wrote: > > > > My concern is that some critical bug will show up at some point > > that requires double-argument kfree_rcu() be invoked while holding > > a raw spinlock. (Single-argument kfree_rcu() must sometimes invoke > > synchronize_rcu(), so it can never be invoked in any state forbidding > > invoking schedule().) > > So you are saying as of today we are good but in near future the > following > synchronize_rcu() -> kfree_rcu() > > may be needed? You lost me on this one. I am instead concerned that something like this might be needed on short notice: raw_spin_lock(&some_lock); kfree_rcu(some_pointer, some_field_offset); In contrast, single-argument kfree_rcu() cannot be invoked from any environment where synchronize_rcu() cannot be invoked. > > Yes, dropping to a plain spinlock would be simple in the here and now, > > but experience indicates that it is only a matter of time, and that when > > that time comes it will come as an emergency. > > Hmmm. I point out the call_rcu() experience. > > One approach would be to replace the "IS_ENABLED(CONFIG_PREEMPT_RT)" > > with some sort of check for being in a context where spinlock acquisition > > is not legal. What could be done along those lines? > > I would rethink the whole concept how this is implemented now and give > it another try. The code does not look pretty and is looking > complicated. The RT covering of this part then just added a simple > return because nothing else seemed to be possible. This patch here > looks like another duct tape attempt to avoid a warning. In addition to the possibility of invocation from BH? Thanx, Paul