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    SubjectRe: 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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