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=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 5E1DBC18E7D for ; Wed, 22 May 2019 09:04:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3DA7D205ED for ; Wed, 22 May 2019 09:04:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728899AbfEVJEi (ORCPT ); Wed, 22 May 2019 05:04:38 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:45462 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728536AbfEVJEi (ORCPT ); Wed, 22 May 2019 05:04:38 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id A043A374; Wed, 22 May 2019 02:04:37 -0700 (PDT) Received: from [10.162.43.129] (p8cg001049571a15.blr.arm.com [10.162.43.129]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A4AC23F718; Wed, 22 May 2019 02:04:34 -0700 (PDT) Subject: Re: [PATCH] arm64: break while loop if task had been rescheduled To: Tengfei Fan , catalin.marinas@arm.com, will.deacon@arm.com Cc: mark.rutland@arm.com, marc.zyngier@arm.com, andreyknvl@google.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, tengfei@codeaurora.org References: <1558430404-4840-1-git-send-email-tengfeif@codeaurora.org> From: Anshuman Khandual Message-ID: Date: Wed, 22 May 2019 14:34:46 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <1558430404-4840-1-git-send-email-tengfeif@codeaurora.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/21/2019 02:50 PM, Tengfei Fan wrote: > While printing a task's backtrace and this task isn't > current task, it is possible that task's fp and fp+8 > have the same value, so cannot break the while loop. > This can break while loop if this task had been > rescheduled during print this task's backtrace. This is very confusing. IIUC it suggests that while printing the backtrace for non-current tasks the do/while loop does not exit because fp and fp+8 might have the same value ? When would this happen ? Even in that case the commit message here does not properly match the change in this patch. This patch tries to stop printing the stack for non-current tasks if their state change while there is one dump_backtrace() trying to print back trace. Dont we have any lock preventing a task in this situation (while dumping it's backtrace) from running again or changing state. > > Signed-off-by: Tengfei Fan > --- > arch/arm64/kernel/traps.c | 23 +++++++++++++++++++++++ > 1 file changed, 23 insertions(+) > > diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c > index 2975598..9df6e02 100644 > --- a/arch/arm64/kernel/traps.c > +++ b/arch/arm64/kernel/traps.c > @@ -103,6 +103,9 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk) > { > struct stackframe frame; > int skip = 0; > + long cur_state = 0; > + unsigned long cur_sp = 0; > + unsigned long cur_fp = 0; > > pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk); > > @@ -127,6 +130,9 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk) > */ > frame.fp = thread_saved_fp(tsk); > frame.pc = thread_saved_pc(tsk); > + cur_state = tsk->state; > + cur_sp = thread_saved_sp(tsk); > + cur_fp = frame.fp; Should 'saved_state|sp|fp' instead as its applicable to non-current tasks only. > } > #ifdef CONFIG_FUNCTION_GRAPH_TRACER > frame.graph = 0; > @@ -134,6 +140,23 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk) > > printk("Call trace:\n"); > do { > + if (tsk != current && (cur_state != tsk->state > + /* > + * We would not be printing backtrace for the task > + * that has changed state from uninterruptible to > + * running before hitting the do-while loop but after > + * saving the current state. If task is in running This does not check any explicit task states like 'un-interruptible' or 'running' but instead tracks change from any previously 'saved' state. > + * state before saving the state, then we may print > + * wrong call trace or end up in infinite while loop > + * if *(fp) and *(fp+8) are same. While the situation Then dump_backtrace() must detect it, should not save it and just abort. > + * will stop print when that task schedule out. Thats not a reliable solution. AFICS we should not proceed further if there is a chance of an wrong trace or an infinite loop. Hoping that the printing will stop when task gets scheduled out does not seem right. > + */ > + || cur_sp != thread_saved_sp(tsk) > + || cur_fp != thread_saved_fp(tsk))) { Why does any of these three mismatches detect the problematic transition not just the state ?