Re: [PATCH v3] tracing: Fix rcu splat from idle CPU on boot.

[Philipp Schrader]

On Mon, Dec 14, 2015 at 11:07 AM, Philipp Schrader
<philipp@xxxxxxxxxxxxxxxx> wrote:
> With PREEMPT_RT and most of the lockdep-related options enabled I
> encountered this splat when booting our DRA7 evaluation module:
>
> [    0.055073]
> [    0.055076] ===============================
> [    0.055079] [ INFO: suspicious RCU usage. ]
> [    0.055084] 4.1.6+ #2 Not tainted
> [    0.055086] -------------------------------
> [    0.055090] include/trace/events/hist.h:31 suspicious
> rcu_dereference_check() usage!
> [    0.055093]
> [    0.055093] other info that might help us debug this:
> [    0.055093]
> [    0.055097]
> [    0.055097] RCU used illegally from idle CPU!
> [    0.055097] rcu_scheduler_active = 1, debug_locks = 1
> [    0.055100] RCU used illegally from extended quiescent state!
> [    0.055104] no locks held by swapper/0/0.
> [    0.055106]
> [    0.055106] stack backtrace:
> [    0.055112] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.1.6+ #2
> [    0.055116] Hardware name: Generic DRA74X (Flattened Device Tree)
> [    0.055130] [<c00196b8>] (unwind_backtrace) from [<c001515c>]
> (show_stack+0x20/0x24)
> [    0.055146] [<c001515c>] (show_stack) from [<c07bc408>]
> (dump_stack+0x84/0xa0)
> [    0.055160] [<c07bc408>] (dump_stack) from [<c009bc38>]
> (lockdep_rcu_suspicious+0xb0/0x110)
> [    0.055172] [<c009bc38>] (lockdep_rcu_suspicious) from [<c01246c4>]
> (time_hardirqs_off+0x2b8/0x3c8)
> [    0.055184] [<c01246c4>] (time_hardirqs_off) from [<c009a218>]
> (trace_hardirqs_off_caller+0x2c/0xf4)
> [    0.055194] [<c009a218>] (trace_hardirqs_off_caller) from
> [<c009a2f4>] (trace_hardirqs_off+0x14/0x18)
> [    0.055204] [<c009a2f4>] (trace_hardirqs_off) from [<c00c7ecc>]
> (rcu_idle_enter+0x78/0xcc)
> [    0.055213] [<c00c7ecc>] (rcu_idle_enter) from [<c0093eb0>]
> (cpu_startup_entry+0x190/0x518)
> [    0.055222] [<c0093eb0>] (cpu_startup_entry) from [<c07b95b4>]
> (rest_init+0x13c/0x17c)
> [    0.055231] [<c07b95b4>] (rest_init) from [<c0b32c74>]
> (start_kernel+0x320/0x380)
> [    0.055238] [<c0b32c74>] (start_kernel) from [<8000807c>] (0x8000807c)
>
> As per Steve Rotstedt's suggestion I changed the trace_* calls to
> trace_*_rcuidle calls. He pointed out that the trace points were getting
> triggered when rcu wasn't watching.
>
> Signed-off-by: Philipp Schrader <philipp@xxxxxxxxxxxxxxxx>
> Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
>  kernel/trace/trace_irqsoff.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/trace/trace_irqsoff.c b/kernel/trace/trace_irqsoff.c
> index aaade2e..d0e1d0e 100644
> --- a/kernel/trace/trace_irqsoff.c
> +++ b/kernel/trace/trace_irqsoff.c
> @@ -450,7 +450,7 @@ EXPORT_SYMBOL_GPL(stop_critical_timings);
>  #ifdef CONFIG_PROVE_LOCKING
>  void time_hardirqs_on(unsigned long a0, unsigned long a1)
>  {
> -       trace_preemptirqsoff_hist(IRQS_ON, 0);
> +       trace_preemptirqsoff_hist_rcuidle(IRQS_ON, 0);
>         if (!preempt_trace() && irq_trace())
>                 stop_critical_timing(a0, a1);
>  }
> @@ -459,7 +459,7 @@ void time_hardirqs_off(unsigned long a0, unsigned long a1)
>  {
>         if (!preempt_trace() && irq_trace())
>                 start_critical_timing(a0, a1);
> -       trace_preemptirqsoff_hist(IRQS_OFF, 1);
> +       trace_preemptirqsoff_hist_rcuidle(IRQS_OFF, 1);
>  }
>
>  #else /* !CONFIG_PROVE_LOCKING */
> --
> 2.1.4
>

Including Paul E. McKenney and Sebastian Andrzej Siewior.
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