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=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 B86E5C2BB86 for ; Tue, 14 Apr 2020 12:55:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A2B16208E0 for ; Tue, 14 Apr 2020 12:55:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2502367AbgDNMzE (ORCPT ); Tue, 14 Apr 2020 08:55:04 -0400 Received: from foss.arm.com ([217.140.110.172]:54860 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2502340AbgDNMy6 (ORCPT ); Tue, 14 Apr 2020 08:54:58 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 2209630E; Tue, 14 Apr 2020 05:54:58 -0700 (PDT) Received: from C02TD0UTHF1T.local (unknown [10.57.30.4]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 1966E3F73D; Tue, 14 Apr 2020 05:54:56 -0700 (PDT) Date: Tue, 14 Apr 2020 13:54:54 +0100 From: Mark Rutland To: Wang Qing Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, opensource.kernel@vivo.com Subject: Re: [PATCH] ARM64: fixed dump_backtrace() when task running on another cpu Message-ID: <20200414125454.GE2486@C02TD0UTHF1T.local> References: <1586425106-7369-1-git-send-email-wangqing@vivo.com> <1586769788-5954-1-git-send-email-wangqing@vivo.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1586769788-5954-1-git-send-email-wangqing@vivo.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Apr 13, 2020 at 05:23:08PM +0800, Wang Qing wrote: > >Hi, > > > >On Thu, Apr 09, 2020 at 05:38:16PM +0800, Wang Qing wrote: > >> We cannot get FP and PC when the task is running on another CPU, > >> task->thread.cpu_context is the last time the task was switched out, > >> it's better to give a reminder than to provide wrong information. > >> > >> Signed-off-by: Wang Qing > > > >Are you seeing this happen anywhere in particular today? > > This problem is not so obvious, because it will not cause any exceptions > but will show "old" stack always ending with switch_to, I finally confirmed > the problem through debugging. > > For example:Task blocked in spinlock/interrupt/busy loop, I want to print > the backtrace when detected(like PSI in Android), the printing is wrong(old). Sure, but *where* are you seeing this? Is this a problem in mainline, or only in code that you add? > > > >> --- > >> arch/arm64/kernel/traps.c | 8 ++++++++ > >> 1 file changed, 8 insertions(+) > >> > >> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c > >> index cf402be..c04e3e8 100644 > >> --- a/arch/arm64/kernel/traps.c > >> +++ b/arch/arm64/kernel/traps.c > >> @@ -106,6 +106,14 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk) > >> start_backtrace(&frame, > >> (unsigned long)__builtin_frame_address(0), > >> (unsigned long)dump_backtrace); > >> + } else if (tsk->on_cpu) { > >> + /* > >> + * The task is running in another cpu, so the call stack > >> + * is changing and we cannot get it. > >> + */ > >> + pr_warn("tsk: %s is running in CPU%d, Don't call trace!\n", > >> + tsk->comm, tsk->cpu); > > > >I believe that we can race with a concurrent write to tsk->cpu in both > >cases above. We could use READ_ONCE() to get a snapshot, but we can > >still race and miss cases where the task was runnning as we backtrace > >it. > > > >Thanks, > >Mark. > > I will use task_cpu(tsk) instead of tsk->cpu, and add task_running_oncpu() in > include/linux/sched.h instead of tsk->on_cpu, but as you said, by this patch, > we can still race and miss cases where the task was runnning as we backtrace. > > But from the user's perspective, printing wrong backtrace is confused when > we call this function while task already running. However, it's reasonable to > print the last backtrace when task enter running during the function is called. The contract of dump_backtrace() is that it's only called for either: * the current task * a task that is blocked in switch_to() ... so I don't think that the current behaviour is wrong as such, though if it's easy to catch misuse I agree it would be nice to do so for robustness. However, we can alwatys race here, so detecting this case is best-effort and not entirely reliable, and I don't want to leave the impression that it is. I'm also not a fan of pr_warn() here, since this is indicative of a kernel bug rather than a user/system issue, and can spam the console due to a lack of ratelimiting. So, based on whether this is a problem in existing code, I'd like that code fix first, and then we can consider adding a WARN_ONCE() or something ratelimited. Ideally something that'll give us the bactrace or the code that's calling dump_backtrace() erroneously. Thanks, Mark.