[PATCH ftrace/core] tracing: irqsoff: Account for additional preempt_disable

[Joel Fernandes (Google)]

Recently we tried to make the preemptirqsoff tracer to use irqsoff
tracepoint probes. However this causes issues as reported by Masami:

[2.271078] Testing tracer preemptirqsoff: .. no entries found ..FAILED!
[2.381015] WARNING: CPU: 0 PID: 1 at /home/mhiramat/ksrc/linux/kernel/
trace/trace.c:1512 run_tracer_selftest+0xf3/0x154

This is due to the tracepoint code increasing the preempt nesting count
by calling an additional preempt_disable before calling into the
preemptoff tracer which messes up the preempt_count() check in
tracer_hardirqs_off.

To fix this, make the irqsoff tracer probes balance the additional outer
preempt_disable with a preempt_enable_notrace.

The other way to fix this is to just use SRCU for all tracepoints.
However we can't do that because we can't use NMIs from RCU context.

Fixes: c3bc8fd637a9 ("tracing: Centralize preemptirq tracepoints
                      and unify their usage")
Fixes: e6753f23d961 ("tracepoint: Make rcuidle tracepoint callers use SRCU")
Reported-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
 kernel/trace/trace_irqsoff.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/kernel/trace/trace_irqsoff.c b/kernel/trace/trace_irqsoff.c
index 770cd30cda40..ffbf1505d5bc 100644
--- a/kernel/trace/trace_irqsoff.c
+++ b/kernel/trace/trace_irqsoff.c
@@ -603,14 +603,40 @@ static void irqsoff_tracer_stop(struct trace_array *tr)
  */
 static void tracer_hardirqs_on(void *none, unsigned long a0, unsigned long a1)
 {
+	/*
+	 * Tracepoint probes are expected to be called with preempt disabled,
+	 * We don't care about being called with preempt disabled but we need
+	 * to know in the future if that changes so we can remove the next
+	 * preempt_enable.
+	 */
+	WARN_ON_ONCE(!preempt_count());
+
+	/* Tracepoint probes disable preemption atleast once, account for that */
+	preempt_enable_notrace();
+
 	if (!preempt_trace() && irq_trace())
 		stop_critical_timing(a0, a1);
+
+	preempt_disable_notrace();
 }
 
 static void tracer_hardirqs_off(void *none, unsigned long a0, unsigned long a1)
 {
+	/*
+	 * Tracepoint probes are expected to be called with preempt disabled,
+	 * We don't care about being called with preempt disabled but we need
+	 * to know in the future if that changes so we can remove the next
+	 * preempt_enable.
+	 */
+	WARN_ON_ONCE(!preempt_count());
+
+	/* Tracepoint probes disable preemption atleast once, account for that */
+	preempt_enable_notrace();
+
 	if (!preempt_trace() && irq_trace())
 		start_critical_timing(a0, a1);
+
+	preempt_disable_notrace();
 }
 
 static int irqsoff_tracer_init(struct trace_array *tr)
-- 
2.18.0.597.ga71716f1ad-goog