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=-1.0 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 426F9C76188 for ; Sat, 20 Jul 2019 07:55:14 +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 0B3DE2184E for ; Sat, 20 Jul 2019 07:55:14 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="m4Yv6Ssc"; dkim=fail reason="signature verification failed" (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="Q53+vJzO" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0B3DE2184E Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org 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:Mime-Version:References:In-Reply-To: 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=8XYfhluU9XSSCNrl7UBqyCnYIcnxkddvnTOYVCgc4ko=; b=m4Yv6Ssc6cj4hs 1g0uKqDrU/5zX0xcNsZxkkD6jYAERwCexj+Jb0LP1WcXA5HVvl/0c2QAejmkB+Km6rvKUBuUHDhib 7WEvUAjYxFsAR0HioWoVGENNIDoddYQlFm7PNCj1G/rgAw1xbdz6XBWZYvFB4Tyl6mYqBQ8OAtxvY 0h8jWnr4a1QxShHLmSwSAVmcq4/g9Z0OCF/oOgD0Dlw0XDrftDSWrA8+drenwuZXqeBRIuPv52RnF tQa3ILh5WH4+pgB/eV39wJib8h173dOlBXo9MeTwwG+tY/AETUO7vCAKtOWoCFIIC7CYo/RkrWqwl cHpFfrdjtjtYXAvI4uOQ==; 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 1hokD1-0008Cp-DW; Sat, 20 Jul 2019 07:55:11 +0000 Received: from mail.kernel.org ([198.145.29.99]) by bombadil.infradead.org with esmtps (Exim 4.92 #3 (Red Hat Linux)) id 1hokCx-0007WL-LD for linux-arm-kernel@lists.infradead.org; Sat, 20 Jul 2019 07:55:09 +0000 Received: from devnote2 (72.65.214.202.bf.2iij.net [202.214.65.72]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 9FF552084C; Sat, 20 Jul 2019 07:55:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1563609304; bh=c/qcAjooUS+XTsmTRW+jDhrjLGqeN6OgrxYeSB1F2M8=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=Q53+vJzOimnDeGgWyVZmYWxjmNNd7QWjAXgV8FUwBw9FZzRKqtNbdE1oxIDPBib5J NW0eJH2epYH4GvO6jdUpcjmSOtJpQrm3oX/qGhKMnk6IB40vJdAZzdTgNJaRJqFrFK i1lY0g8c0blNVYVjwEwQSZWyN1rA0dijHyn+HXDk= Date: Sat, 20 Jul 2019 16:54:58 +0900 From: Masami Hiramatsu To: Mark Rutland Subject: Re: [PATCH 3/3] arm64: debug: Remove rcu_read_lock from debug exception Message-Id: <20190720165458.7333b65244312843c2ca6857@kernel.org> In-Reply-To: <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> <20190719095958.GA19605@lakrids.cambridge.arm.com> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.32; x86_64-pc-linux-gnu) Mime-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190720_005507_738300_8F34071F X-CRM114-Status: GOOD ( 19.51 ) 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 Hi Mark, On Fri, 19 Jul 2019 10:59:59 +0100 Mark Rutland wrote: > 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). On x86, it uses rcu_nmi_enter/exit() for kernel mode. For user mode, we don't need to care since it must not be an idle task. Thank you, -- Masami Hiramatsu _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel