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=-2.2 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 CAE14C76188 for ; Fri, 19 Jul 2019 10:00:29 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 9F5982173B for ; Fri, 19 Jul 2019 10:00:29 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="sqDjKy1R" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9F5982173B Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=IqCShs+uHn01rnax/HMlFQJM8yMXOrrERG3qBa2Nyqg=; b=sqDjKy1R5cZuRs 1DXtk/mnGfVda3MdXhop9PQD3k9PrSa+zCapss4UJaydG2fHgTSgbUpgqQuwrQQXp/yuHwYvyrZnW m5WQhvkbQDyGS789c1fWraehEf0TM/wp7Gj4EYqlR/QY/GCYe5uWVSjAuWci53t/KaZgr4YOyRIzq IlBgCHFeur5oFpurRI/ZoZcwsvK07VUo/ZtJRMNZ6gZC/dZQx6xeZd9Hc1DlJG2kpMd8x6yI5wjdg v9r8Fxmzl3Pv5r3rzNNkj/RI9ERtpeNJsxDUOYvenbtkJYNMEJh6z/dwuRjxXfFP+SPjXkG9ZTgYI CuWnjFrkl3C8JnfJ828g==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92 #3 (Red Hat Linux)) id 1hoPgT-0001jI-Ma; Fri, 19 Jul 2019 10:00:13 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.92 #3 (Red Hat Linux)) id 1hoPgQ-0001M2-3G for linux-arm-kernel@lists.infradead.org; Fri, 19 Jul 2019 10:00:11 +0000 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 DAF05337; Fri, 19 Jul 2019 03:00:05 -0700 (PDT) Received: from lakrids.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 4AC3F3F59C; Fri, 19 Jul 2019 03:00:04 -0700 (PDT) Date: Fri, 19 Jul 2019 10:59:59 +0100 From: Mark Rutland To: Masami Hiramatsu Subject: Re: [PATCH 3/3] arm64: debug: Remove rcu_read_lock from debug exception Message-ID: <20190719095958.GA19605@lakrids.cambridge.arm.com> References: <156342860634.8565.14804606041960884732.stgit@devnote2> <156342863822.8565.7624877983728871995.stgit@devnote2> <20190718062215.GG14271@linux.ibm.com> <20190718092022.GA3625@blommer> <20190718233133.146065f668da6297e57e52ef@kernel.org> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20190718233133.146065f668da6297e57e52ef@kernel.org> User-Agent: Mutt/1.11.1+11 (2f07cb52) (2018-12-01) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190719_030010_230993_BCE1C6A6 X-CRM114-Status: GOOD ( 16.86 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Dan Rue , Daniel Diaz , Anders Roxell , Catalin Marinas , Naresh Kamboju , Will Deacon , linux-kernel@vger.kernel.org, James Morse , Matt Hart , "Paul E. McKenney" , linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Jul 18, 2019 at 11:31:33PM +0900, Masami Hiramatsu wrote: > On Thu, 18 Jul 2019 10:20:23 +0100 > Mark Rutland wrote: > > > On Wed, Jul 17, 2019 at 11:22:15PM -0700, Paul E. McKenney wrote: > > > On Thu, Jul 18, 2019 at 02:43:58PM +0900, Masami Hiramatsu wrote: > > > > Remove rcu_read_lock()/rcu_read_unlock() from debug exception > > > > handlers since the software breakpoint can be hit on idle task. > > > > Why precisely do we need to elide these? Are we seeing warnings today? > > Yes, unfortunately, or fortunately. Naresh reported that warns when > ftracetest ran. I confirmed that happens if I probe on default_idle_call too. > > /sys/kernel/debug/tracing # echo p default_idle_call >> kprobe_events > /sys/kernel/debug/tracing # echo 1 > events/kprobes/enable > /sys/kernel/debug/tracing # [ 135.122237] > [ 135.125035] ============================= > [ 135.125310] WARNING: suspicious RCU usage > [ 135.125581] 5.2.0-08445-g9187c508bdc7 #20 Not tainted > [ 135.125904] ----------------------------- > [ 135.126205] include/linux/rcupdate.h:594 rcu_read_lock() used illegally while idle! > [ 135.126839] > [ 135.126839] other info that might help us debug this: > [ 135.126839] > [ 135.127410] > [ 135.127410] RCU used illegally from idle CPU! > [ 135.127410] rcu_scheduler_active = 2, debug_locks = 1 > [ 135.128114] RCU used illegally from extended quiescent state! > [ 135.128555] 1 lock held by swapper/0/0: > [ 135.128944] #0: (____ptrval____) (rcu_read_lock){....}, at: call_break_hook+0x0/0x178 > [ 135.130499] > [ 135.130499] stack backtrace: > [ 135.131192] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.2.0-08445-g9187c508bdc7 #20 > [ 135.131841] Hardware name: linux,dummy-virt (DT) > [ 135.132224] Call trace: > [ 135.132491] dump_backtrace+0x0/0x140 > [ 135.132806] show_stack+0x24/0x30 > [ 135.133133] dump_stack+0xc4/0x10c > [ 135.133726] lockdep_rcu_suspicious+0xf8/0x108 > [ 135.134171] call_break_hook+0x170/0x178 > [ 135.134486] brk_handler+0x28/0x68 > [ 135.134792] do_debug_exception+0x90/0x150 > [ 135.135051] el1_dbg+0x18/0x8c > [ 135.135260] default_idle_call+0x0/0x44 > [ 135.135516] cpu_startup_entry+0x2c/0x30 > [ 135.135815] rest_init+0x1b0/0x280 > [ 135.136044] arch_call_rest_init+0x14/0x1c > [ 135.136305] start_kernel+0x4d4/0x500 > [ 135.136597] > > > > The exception entry and exit use irq_enter() and irq_exit(), in this > > > case, correct? Otherwise RCU will be ignoring this CPU. > > > > This is missing today, which sounds like the underlying bug. > > Agreed. I'm not so familier with how debug exception is handled on arm64, > would it be a kind of NMI or IRQ? They're more like faults, in that they're synchronous exceptions. Given that, I think using irq_enter() / irq_exit() would be surprising here, but perhaps they're misnamed. What do other architectures do here? Having a kprobe on the critical path to idle doesn't sound specific to arm64, but perhaps it is (and we should rule it out). Thanks, Mark. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel