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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id A7F45ECAAA3 for ; Fri, 26 Aug 2022 03:25:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244449AbiHZDZv (ORCPT ); Thu, 25 Aug 2022 23:25:51 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50798 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233064AbiHZDZp (ORCPT ); Thu, 25 Aug 2022 23:25:45 -0400 Received: from szxga08-in.huawei.com (szxga08-in.huawei.com [45.249.212.255]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 902FFBC804 for ; Thu, 25 Aug 2022 20:25:42 -0700 (PDT) Received: from dggpemm500021.china.huawei.com (unknown [172.30.72.55]) by szxga08-in.huawei.com (SkyGuard) with ESMTP id 4MDQ9r3GR0z1N7MS; Fri, 26 Aug 2022 11:22:08 +0800 (CST) Received: from dggpemm500006.china.huawei.com (7.185.36.236) by dggpemm500021.china.huawei.com (7.185.36.109) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Fri, 26 Aug 2022 11:25:40 +0800 Received: from [10.174.178.55] (10.174.178.55) by dggpemm500006.china.huawei.com (7.185.36.236) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Fri, 26 Aug 2022 11:25:40 +0800 Subject: Re: [PATCH 1/2] arm64/traps: Replace this_cpu_* with raw_cpu_* in panic_bad_stack() To: Mark Rutland CC: Catalin Marinas , Will Deacon , Russell King , , , References: <20220825063154.69-1-thunder.leizhen@huawei.com> <20220825063154.69-2-thunder.leizhen@huawei.com> From: "Leizhen (ThunderTown)" Message-ID: <569fd51d-aca2-f1c0-d90e-2ae34e4f6f99@huawei.com> Date: Fri, 26 Aug 2022 11:25:39 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.178.55] X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) To dggpemm500006.china.huawei.com (7.185.36.236) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022/8/25 21:29, Mark Rutland wrote: > On Thu, Aug 25, 2022 at 02:31:53PM +0800, Zhen Lei wrote: >> The hardware automatically disable the IRQ interrupt before jumping to the >> interrupt or exception vector. Therefore, the preempt_disable() operation >> in this_cpu_read() after macro expansion is unnecessary. In fact, before >> commit 8168f098867f ("arm64: entry: split bad stack entry"), the operation >> this_cpu_read() precedes arm64_enter_nmi(). If set_preempt_need_resched() >> is called before stack overflow, this_cpu_read() may trigger scheduling, >> see pseudocode below. >> >> Pseudocode of this_cpu_read(xx) when CONFIG_PREEMPTION=y: >> preempt_disable_notrace(); >> raw_cpu_read(xx); >> if (unlikely(__preempt_count_dec_and_test())) >> __preempt_schedule_notrace(); > > Ok, but in mainline we have commit 8168f098867f; so we cannot reach here > without having fiddled with the preempt count. > > Are you saying that some stable kernel is broken because it lacks commit > 8168f098867f? Is so, I think the right fix is to backport commit 8168f098867f, > and that is then irrelevant to this change. Yes, after backport commit 8168f098867f, the risk is gone. > >> Therefore, use raw_cpu_* instead of this_cpu_* to eliminate potential >> hazards. At the very least, it reduces a few lines of assembly code. > > I'm happy to use raw_cpu_*() here, to minimize the work we have to do, any any > risks with e.g. instrumentation, but as above I don't think the case mentioned > in the commit message is relevant. OK, I will delete the description about risk. > > Thanks, > Mark. > >> >> Signed-off-by: Zhen Lei >> --- >> arch/arm64/kernel/traps.c | 4 ++-- >> 1 file changed, 2 insertions(+), 2 deletions(-) >> >> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c >> index b7fed33981f7b76..e6b6f4650e3d895 100644 >> --- a/arch/arm64/kernel/traps.c >> +++ b/arch/arm64/kernel/traps.c >> @@ -871,8 +871,8 @@ DEFINE_PER_CPU(unsigned long [OVERFLOW_STACK_SIZE/sizeof(long)], overflow_stack) >> void panic_bad_stack(struct pt_regs *regs, unsigned long esr, unsigned long far) >> { >> unsigned long tsk_stk = (unsigned long)current->stack; >> - unsigned long irq_stk = (unsigned long)this_cpu_read(irq_stack_ptr); >> - unsigned long ovf_stk = (unsigned long)this_cpu_ptr(overflow_stack); >> + unsigned long irq_stk = (unsigned long)raw_cpu_read(irq_stack_ptr); >> + unsigned long ovf_stk = (unsigned long)raw_cpu_ptr(overflow_stack); >> >> console_verbose(); >> pr_emerg("Insufficient stack space to handle exception!"); >> -- >> 2.25.1 >> > . > -- Regards, Zhen Lei