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Subject[PATCH] tracing: Fix rcu splat from idle CPU on boot.
Date
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.
---
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


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