[PATCH 4.9 34/54] ftrace: Move RCU is watching check after recursion check

[Greg Kroah-Hartman]

From: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>

commit b40341fad6cc2daa195f8090fd3348f18fff640a upstream.

The first thing that the ftrace function callback helper functions should do
is to check for recursion. Peter Zijlstra found that when
"rcu_is_watching()" had its notrace removed, it caused perf function tracing
to crash. This is because the call of rcu_is_watching() is tested before
function recursion is checked and and if it is traced, it will cause an
infinite recursion loop.

rcu_is_watching() should still stay notrace, but to prevent this should
never had crashed in the first place. The recursion prevention must be the
first thing done in callback functions.

Link: https://lore.kernel.org/r/20200929112541.GM2628@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Cc: stable@xxxxxxxxxxxxxxx
Cc: Paul McKenney <paulmck@xxxxxxxxxx>
Fixes: c68c0fa293417 ("ftrace: Have ftrace_ops_get_func() handle RCU and PER_CPU flags too")
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reported-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/trace/ftrace.c |    8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -5334,17 +5334,15 @@ static void ftrace_ops_assist_func(unsig
 {
 	int bit;
 
-	if ((op->flags & FTRACE_OPS_FL_RCU) && !rcu_is_watching())
-		return;
-
 	bit = trace_test_and_set_recursion(TRACE_LIST_START, TRACE_LIST_MAX);
 	if (bit < 0)
 		return;
 
 	preempt_disable_notrace();
 
-	if (!(op->flags & FTRACE_OPS_FL_PER_CPU) ||
-	    !ftrace_function_local_disabled(op)) {
+	if ((!(op->flags & FTRACE_OPS_FL_RCU) || rcu_is_watching()) &&
+	    (!(op->flags & FTRACE_OPS_FL_PER_CPU) ||
+	     !ftrace_function_local_disabled(op))) {
 		op->func(ip, parent_ip, op, regs);
 	}