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=-5.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_2 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 6AF5AC433E1 for ; Thu, 13 Aug 2020 15:37:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3793820829 for ; Thu, 13 Aug 2020 15:37:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726676AbgHMPhs (ORCPT ); Thu, 13 Aug 2020 11:37:48 -0400 Received: from mail.kernel.org ([198.145.29.99]:44532 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726249AbgHMPhr (ORCPT ); Thu, 13 Aug 2020 11:37:47 -0400 Received: from oasis.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (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 36A4E20781; Thu, 13 Aug 2020 15:37:45 +0000 (UTC) Date: Thu, 13 Aug 2020 11:37:43 -0400 From: Steven Rostedt To: Palmer Dabbelt Cc: guoren@kernel.org, Paul Walmsley , mingo@redhat.com, linux-kernel@vger.kernel.org, linux-csky@vger.kernel.org, guoren@linux.alibaba.com, linux-riscv@lists.infradead.org Subject: Re: [PATCH] ftrace: Fixup lockdep assert held of text_mutex Message-ID: <20200813113743.001b6c31@oasis.local.home> In-Reply-To: References: X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-csky-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-csky@vger.kernel.org On Wed, 12 Aug 2020 22:13:19 -0700 (PDT) Palmer Dabbelt wrote: > Sorry, I'm not really sure what's going on here. I'm not really seeing code > that matches this in our port right now, so maybe this is aginst some other > tree? If it's the RISC-V kprobes patch set then I was hoping to take a look at > that tomorrow (or I guess a bit earlier this week, but I had some surprise work > stuff to do). IIRC there were a handful of races in the last patch set I saw, > but it's been a while so I don't remember for sure. > > That said, I certainly wouldn't be surprised if there's a locking bug in our > ftrace stuff. It'd be way easier for me to figure out what's going on if you > have a concrete suggestion as to how to fix the issues -- even if it's just a > workaround. The issue is actually quite basic. ftrace_init_nop() is called quite early in boot up and never called again. It's called before SMP is set up, so it's on a single CPU, and no worries about synchronization with other CPUs is needed. On x86, it is called before text_poke() is initialized (which is used to synchronize code updates across CPUs), and thus can't be called. There's a "text_poke_early()" that is used instead, which is basically just a memcpy(). Now, if ftrace_init_nop() is not defined by the architecture, it is a simple call to ftrace_make_nop(), which is also used to disable ftrace callbacks. The issue is that we have the following path on riscv: ftrace_init_nop() ftrace_make_nop() __ftrace_modify_call() patch_text_nosync() patch_insn_write() lockdep_assert_held(&text_mutex); Boom! text_mutex is not held, and lockdep complains. The difference between ftrace_make_nop() being called by ftrace_init_nop() and being called later to disable function tracing is that the latter will have: ftrace_arch_code_modify_prepare(); [..] ftrace_make_nop(); [..] ftrace_arch_code_modify_post_process(); and the former will not have those called. On x86, we handle the two different cases with: static int ftrace_poke_late = 0; int ftrace_arch_code_modify_prepare(void) { mutex_lock(&text_mutex); ftrace_poke_late = 1; return 0; } int ftrace_arch_code_modify_post_process(void) { text_poke_finish(); ftrace_poke_late = 0; mutex_unlock(&text_mutex); } Although, the post_process() probably doesn't even need to set ftrace_poke_late back to zero. Then in ftrace_make_nop(), we have: ftrace_make_nop() ftrace_modify_code_direct() if (ftrace_poke_late) text_poke_queue(...); // this checks if text_mutex is held else text_poke_early(...); // is basically just memcpy, no test on text_mutex. The two solutions for riscv, is either to implement the same thing as above, or you can create your own ftrace_init_nop() to take the text_mutex before calling ftrace_make_nop(), and that should work too. -- Steve