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 86FF8ECAAD2 for ; Fri, 26 Aug 2022 06:22:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244993AbiHZGWn (ORCPT ); Fri, 26 Aug 2022 02:22:43 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44868 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S244977AbiHZGWj (ORCPT ); Fri, 26 Aug 2022 02:22:39 -0400 Received: from szxga08-in.huawei.com (szxga08-in.huawei.com [45.249.212.255]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6FC2DD075D for ; Thu, 25 Aug 2022 23:22:36 -0700 (PDT) Received: from dggpemm500024.china.huawei.com (unknown [172.30.72.56]) by szxga08-in.huawei.com (SkyGuard) with ESMTP id 4MDV5x6bRWz19VbC; Fri, 26 Aug 2022 14:19:01 +0800 (CST) Received: from dggpemm500006.china.huawei.com (7.185.36.236) by dggpemm500024.china.huawei.com (7.185.36.203) 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 14:22:34 +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 14:22:33 +0800 Subject: Re: [PATCH 2/2] ARM: 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> <20220825063154.69-3-thunder.leizhen@huawei.com> From: "Leizhen (ThunderTown)" Message-ID: <7a905d7a-9670-a672-07ca-4e2ed37a35ac@huawei.com> Date: Fri, 26 Aug 2022 14:22:33 +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: dggems703-chm.china.huawei.com (10.3.19.180) 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 21:32, Mark Rutland wrote: > On Thu, Aug 25, 2022 at 02:31:54PM +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, function >> this_cpu_read() may trigger scheduling, see pseudocode below. >> >> Pseudocode of this_cpu_read(xx): >> preempt_disable_notrace(); >> raw_cpu_read(xx); >> if (unlikely(__preempt_count_dec_and_test())) >> __preempt_schedule_notrace(); >> >> 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 think if scheduling is a problem here, something should increment the > preempt_count as is done on arm64, since any other operation in this function > could end up causing preemption. Yes, right. Sorry, I'm stuck in this_cpu_read()'s analysis. > > Regardless, I also think it's sensible to use raw_cpu_*() here, but I don't > think that actually fixes the problem the commit message describes. OK, I will delete the description about risk. The risk I mentioned in the commit message was mainly to show that using raw_cpu_read() would be better than using this_cpu_read() in this case. > > Thanks, > Mark. > >> >> Signed-off-by: Zhen Lei >> --- >> KernelVersion: v6.0-rc2 >> arch/arm/kernel/traps.c | 4 ++-- >> 1 file changed, 2 insertions(+), 2 deletions(-) >> >> diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c >> index 1518a1f443ff866..d5903d790cf3b7e 100644 >> --- a/arch/arm/kernel/traps.c >> +++ b/arch/arm/kernel/traps.c >> @@ -927,9 +927,9 @@ asmlinkage void handle_bad_stack(struct pt_regs *regs) >> { >> unsigned long tsk_stk = (unsigned long)current->stack; >> #ifdef CONFIG_IRQSTACKS >> - unsigned long irq_stk = (unsigned long)this_cpu_read(irq_stack_ptr); >> + unsigned long irq_stk = (unsigned long)raw_cpu_read(irq_stack_ptr); >> #endif >> - unsigned long ovf_stk = (unsigned long)this_cpu_read(overflow_stack_ptr); >> + unsigned long ovf_stk = (unsigned long)raw_cpu_read(overflow_stack_ptr); >> >> console_verbose(); >> pr_emerg("Insufficient stack space to handle exception!"); >> -- >> 2.25.1 >> > . > -- Regards, Zhen Lei