Re: linux-next: stall warnings and deadlock on Arm64 (was: [PATCH] kfence: Avoid stalling...)

Mark Rutland
2020-11-25T10:29:20Z

On Wed, Nov 25, 2020 at 10:45:17AM +0100, Marco Elver wrote:
> On Tue, Nov 24, 2020 at 07:30PM +0000, Mark Rutland wrote:

[...]

> > > I noticed there are a bunch of warnings in the log
> > > that might be relevant (see attached).
> >
> > > [ 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
> >
> > Hmm... I suspect that arch_cpu_idle() is being traced here, and I reckon
> > we have to mark that and its callees as noinstr, since it doesn't seem
> > sane to have ftrace check whether RCU is watching for every function
> > call. Maybe Paul or Steve can correct me. ;)
>
> Yes, it's arch_cpu_idle().
>
> > If you still have the binary lying around, can you check whether
> > default_idle_call+0x94/0x38c is just after the call to arch_cpu_idle()?
> > If you could dump the asm around that, along with whatever faddr2line
> > tells you, that'd be a great help.
>
> I reran to be sure, with similar results. I've attached a
> syz-symbolize'd version of the warnings.

Thanks for confirming, and for the symbolized report.

I'll see about getting this fixed ASAP.

Thanks,
Mark.