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Date: Tue, 24 Nov 2020 19:43:08 +0000 From: Mark Rutland <mark.rutland@....com> To: "Paul E. McKenney" <paulmck@...nel.org> Cc: Marco Elver <elver@...gle.com>, Will Deacon <will@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, Anders Roxell <anders.roxell@...aro.org>, Andrew Morton <akpm@...ux-foundation.org>, Alexander Potapenko <glider@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>, Jann Horn <jannh@...gle.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>, kasan-dev <kasan-dev@...glegroups.com>, rcu@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>, Tejun Heo <tj@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>, linux-arm-kernel@...ts.infradead.org, boqun.feng@...il.com, tglx@...utronix.de Subject: Re: linux-next: stall warnings and deadlock on Arm64 (was: [PATCH] kfence: Avoid stalling...) 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.
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