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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4302FC4332F for ; Wed, 13 Oct 2021 01:53:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 25F4361077 for ; Wed, 13 Oct 2021 01:53:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236627AbhJMBzM (ORCPT ); Tue, 12 Oct 2021 21:55:12 -0400 Received: from out30-130.freemail.mail.aliyun.com ([115.124.30.130]:55501 "EHLO out30-130.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231543AbhJMBzK (ORCPT ); Tue, 12 Oct 2021 21:55:10 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R191e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04357;MF=yun.wang@linux.alibaba.com;NM=1;PH=DS;RN=31;SR=0;TI=SMTPD_---0UrdeUV._1634089981; Received: from testdeMacBook-Pro.local(mailfrom:yun.wang@linux.alibaba.com fp:SMTPD_---0UrdeUV._1634089981) by smtp.aliyun-inc.com(127.0.0.1); Wed, 13 Oct 2021 09:53:02 +0800 Subject: Re: [PATCH 1/2] ftrace: disable preemption on the testing of recursion To: Steven Rostedt , Miroslav Benes Cc: Guo Ren , Ingo Molnar , "James E.J. Bottomley" , Helge Deller , Michael Ellerman , Benjamin Herrenschmidt , Paul Mackerras , Paul Walmsley , Palmer Dabbelt , Albert Ou , Thomas Gleixner , Borislav Petkov , x86@kernel.org, "H. Peter Anvin" , Josh Poimboeuf , Jiri Kosina , Petr Mladek , Joe Lawrence , Colin Ian King , Masami Hiramatsu , "Peter Zijlstra (Intel)" , Nicholas Piggin , Jisheng Zhang , linux-csky@vger.kernel.org, linux-kernel@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, live-patching@vger.kernel.org References: <8c7de46d-9869-aa5e-2bb9-5dbc2eda395e@linux.alibaba.com> <20211012082920.1f8d6557@gandalf.local.home> From: =?UTF-8?B?546L6LSH?= Message-ID: <6ca59cdd-e378-b8c6-7ac6-472568f1af97@linux.alibaba.com> Date: Wed, 13 Oct 2021 09:52:16 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:78.0) Gecko/20100101 Thunderbird/78.14.0 MIME-Version: 1.0 In-Reply-To: <20211012082920.1f8d6557@gandalf.local.home> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/10/12 下午8:29, Steven Rostedt wrote: > On Tue, 12 Oct 2021 14:24:43 +0200 (CEST) > Miroslav Benes wrote: > >>> +++ b/kernel/livepatch/patch.c >>> @@ -52,11 +52,6 @@ static void notrace klp_ftrace_handler(unsigned long ip, >>> bit = ftrace_test_recursion_trylock(ip, parent_ip); >>> if (WARN_ON_ONCE(bit < 0)) >>> return; >>> - /* >>> - * A variant of synchronize_rcu() is used to allow patching functions >>> - * where RCU is not watching, see klp_synchronize_transition(). >>> - */ >>> - preempt_disable_notrace(); >>> >>> func = list_first_or_null_rcu(&ops->func_stack, struct klp_func, >>> stack_node); >>> @@ -120,7 +115,6 @@ static void notrace klp_ftrace_handler(unsigned long ip, >>> klp_arch_set_pc(fregs, (unsigned long)func->new_func); >>> >>> unlock: >>> - preempt_enable_notrace(); >>> ftrace_test_recursion_unlock(bit); >>> } >> >> I don't like this change much. We have preempt_disable there not because >> of ftrace_test_recursion, but because of RCU. ftrace_test_recursion was >> added later. Yes, it would work with the change, but it would also hide >> things which should not be hidden in my opinion. > > Agreed, but I believe the change is fine, but requires a nice comment to > explain what you said above. > > Thus, before the "ftrace_test_recursion_trylock()" we need: > > /* > * The ftrace_test_recursion_trylock() will disable preemption, > * which is required for the variant of synchronize_rcu() that is > * used to allow patching functions where RCU is not watching. > * See klp_synchronize_transition() for more details. > */ Will be in v2 too :-) Regards, Michael Wang > > -- Steve >