[for-next][PATCH 13/18] ftrace: Disable RCU unsafe checker when function graph is enabled

[Steven Rostedt]

From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>

Having the RCU unsafe checker running when function graph is enabled
can cause a live lock. That's because the RCU unsafe checker enables
full lockdep debugging on RCU which does a lot of internal calls that
may be traced by the function graph tracer. This adds quite a bit of
overhead and can possibly live lock the system by causing the timer
interrupt to take longer than 1 HZ.

Just do not do the RCU unsafe checks when function graph tracer is
enabled.

Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 kernel/trace/ftrace.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 9fcf842..899c8c1 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -5118,6 +5118,12 @@ int register_ftrace_graph(trace_func_graph_ret_t retfunc,
 		goto out;
 	}
 
+	/*
+	 * The Unsafe RCU checker can live lock function graph tracing.
+	 * It's best to just disable it while doing the fgraph tracing.
+	 */
+	ftrace_unsafe_rcu_checker_disable();
+
 	ftrace_graph_return = retfunc;
 	ftrace_graph_entry = entryfunc;
 
@@ -5141,6 +5147,7 @@ void unregister_ftrace_graph(void)
 	ftrace_shutdown(&global_ops, FTRACE_STOP_FUNC_RET);
 	unregister_pm_notifier(&ftrace_suspend_notifier);
 	unregister_trace_sched_switch(ftrace_graph_probe_sched_switch, NULL);
+	ftrace_unsafe_rcu_checker_enable();
 
  out:
 	mutex_unlock(&ftrace_lock);
-- 
1.7.10.4


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