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=-4.1 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 0AE8BC433E3 for ; Thu, 16 Jul 2020 15:36:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D809520870 for ; Thu, 16 Jul 2020 15:36:43 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="1hf64CMC"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="mVln4EgB" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729123AbgGPPgm (ORCPT ); Thu, 16 Jul 2020 11:36:42 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:34056 "EHLO galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728774AbgGPPgl (ORCPT ); Thu, 16 Jul 2020 11:36:41 -0400 Date: Thu, 16 Jul 2020 17:36:38 +0200 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1594913800; h=from:from:reply-to:subject:subject: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=7Znin1qZ+GVKKZWfo8pTR1eebjm4kkJGTUgY2Ff7zFs=; b=1hf64CMCc0Fj0miZJXm5fWlr1x+X4rZ0/8IKFuL23AY19Kkitfbe5vN/cU0LDo5C+vAexw KyRrXl1xcF4oOre+ooBVnfK0hmqYVQJiDWQypcBauUFPgJe3+6xrQdd1l5h17ZF+AbdIeJ WfEw/YRuoQqwRne91sT6xtmjDuxrwkuIpTUGQP1n0i6quu9BmuM8eJy2JB5X4Ot0oCQZfJ ZzM/YkNZ2cOSq/gIjkcKtupxrDIueUCTVKq6pX9roQqm8ZzdEAbo1skjYotJMLzpSl69h/ GFN4xb1E7v+/nnbWXCQUAGlloxxRLsLrrKRAof/k//8bSnXcTWsw54kcRSLwHQ== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1594913800; h=from:from:reply-to:subject:subject: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=7Znin1qZ+GVKKZWfo8pTR1eebjm4kkJGTUgY2Ff7zFs=; b=mVln4EgByMSCBqxuFMb+Cgh62v5trn1mfRNiJ5PRxrPIdAwcYPe6VY8njXiz9FDZuvohUy PHqWcj7Y0XUj+7CQ== From: Sebastian Andrzej Siewior To: "Paul E. McKenney" 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: <20200716153638.gfh6dzp2h35ygfaa@linutronix.de> 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> <20200716152027.GQ9247@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200716152027.GQ9247@paulmck-ThinkPad-P72> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020-07-16 08:20:27 [-0700], Paul E. McKenney wrote: > 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. I see. We don't have any kfree() in that context as far as I remember. We had a few cases in "resize" where you allocate memory, copy content and free old memory while under the lock but they are gone. > > > 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? Invocation from BH should be possible because network would probably be the first user. I don't remember anything wrong with BH if I remember correctly. > Thanx, Paul Sebastian