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[23.128.96.18]) by mx.google.com with ESMTP id cd11si270576ejb.245.2020.11.24.15.53.13; Tue, 24 Nov 2020 15:53:36 -0800 (PST) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=arm.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726485AbgKXTnS (ORCPT + 99 others); Tue, 24 Nov 2020 14:43:18 -0500 Received: from foss.arm.com ([217.140.110.172]:51230 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726357AbgKXTnQ (ORCPT ); Tue, 24 Nov 2020 14:43:16 -0500 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 9D2441396; Tue, 24 Nov 2020 11:43:15 -0800 (PST) Received: from C02TD0UTHF1T.local (unknown [10.57.26.92]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id B2D9D3F718; Tue, 24 Nov 2020 11:43:11 -0800 (PST) Date: Tue, 24 Nov 2020 19:43:08 +0000 From: Mark Rutland To: "Paul E. McKenney" Cc: Marco Elver , Will Deacon , Steven Rostedt , Anders Roxell , Andrew Morton , Alexander Potapenko , Dmitry Vyukov , Jann Horn , Linux Kernel Mailing List , Linux-MM , kasan-dev , rcu@vger.kernel.org, Peter Zijlstra , Tejun Heo , Lai Jiangshan , linux-arm-kernel@lists.infradead.org, boqun.feng@gmail.com, tglx@linutronix.de Subject: Re: linux-next: stall warnings and deadlock on Arm64 (was: [PATCH] kfence: Avoid stalling...) Message-ID: <20201124194308.GC8957@C02TD0UTHF1T.local> References: <20201119170259.GA2134472@elver.google.com> <20201119184854.GY1437@paulmck-ThinkPad-P72> <20201119193819.GA2601289@elver.google.com> <20201119213512.GB1437@paulmck-ThinkPad-P72> <20201119225352.GA5251@willie-the-truck> <20201120103031.GB2328@C02TD0UTHF1T.local> <20201120140332.GA3120165@elver.google.com> <20201123193241.GA45639@C02TD0UTHF1T.local> <20201124140310.GA811510@elver.google.com> <20201124150146.GH1437@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201124150146.GH1437@paulmck-ThinkPad-P72> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 24, 2020 at 07:01:46AM -0800, Paul E. McKenney wrote: > On Tue, Nov 24, 2020 at 03:03:10PM +0100, Marco Elver wrote: > > [ 91.184432] ============================= > > [ 91.188301] WARNING: suspicious RCU usage > > [ 91.192316] 5.10.0-rc4-next-20201119-00002-g51c2bf0ac853 #25 Tainted: G W > > [ 91.197536] ----------------------------- > > [ 91.201431] kernel/trace/trace_preemptirq.c:78 RCU not watching trace_hardirqs_off()! > > [ 91.206546] > > [ 91.206546] other info that might help us debug this: > > [ 91.206546] > > [ 91.211790] > > [ 91.211790] rcu_scheduler_active = 2, debug_locks = 0 > > [ 91.216454] RCU used illegally from extended quiescent state! > > [ 91.220890] no locks held by swapper/0/0. > > [ 91.224712] > > [ 91.224712] stack backtrace: > > [ 91.228794] CPU: 0 PID: 0 Comm: swapper/0 Tainted: G W 5.10.0-rc4-next-20201119-00002-g51c2bf0ac853 #25 > > [ 91.234877] Hardware name: linux,dummy-virt (DT) > > [ 91.239032] Call trace: > > [ 91.242587] dump_backtrace+0x0/0x240 > > [ 91.246500] show_stack+0x34/0x88 > > [ 91.250295] dump_stack+0x140/0x1bc > > [ 91.254159] lockdep_rcu_suspicious+0xe4/0xf8 > > [ 91.258332] trace_hardirqs_off+0x214/0x330 > > [ 91.262462] trace_graph_return+0x1ac/0x1d8 > > [ 91.266564] ftrace_return_to_handler+0xa4/0x170 > > [ 91.270809] return_to_handler+0x1c/0x38 > > [ 91.274826] default_idle_call+0x94/0x38c > > [ 91.278869] do_idle+0x240/0x290 > > [ 91.282633] rest_init+0x1e8/0x2dc > > [ 91.286529] arch_call_rest_init+0x1c/0x28 > > [ 91.290585] start_kernel+0x638/0x670 > This looks like tracing in the idle loop in a place where RCU is not > watching. Historically, this has been addressed by using _rcuidle() > trace events, but the portion of the idle loop that RCU is watching has > recently increased. Last I checked, there were still a few holdouts (that > would splat like this) in x86, though perhaps those have since been fixed. Yup! I think this is a latent issue my debug hacks revealed (in addition to a couple of other issues in the idle path), and still affects x86 and others. It's only noticeable if you hack trace_hardirqs_{on,off}() to check rcu_is_watching(), which I had at the tip of my tree. AFAICT, the issue is that arch_cpu_idle() can be dynamically traced with ftrace, and hence the tracing code can unexpectedly run without RCU watching. Since that's dynamic tracing, we can avoid it by marking arch_cpu_idle() and friends as noinstr. I'll see about getting this fixed before we upstream the debug hack. Thanks, Mark.