ftrace: Move RCU is watching check after recursion check

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@hirez.programming.kicks-ass.net
Cc: stable@vger.kernel.org
Cc: Paul McKenney <paulmck@kernel.org>
Fixes: fde22bb307914 ("ftrace: Have ftrace_ops_get_func() handle RCU and PER_CPU flags too")
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reported-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 603255f5f085b867dd273588cad95ad7a61b78cd..541453927c82ad4c8ce3cae2a36e42ea3aae8abb 100644 (file)
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -6993,16 +6993,14 @@ static void ftrace_ops_assist_func(unsigned long ip, unsigned long parent_ip,
 {
        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();
 
-       op->func(ip, parent_ip, op, regs);
+       if (!(op->flags & FTRACE_OPS_FL_RCU) || rcu_is_watching())
+               op->func(ip, parent_ip, op, regs);
 
        preempt_enable_notrace();
        trace_clear_recursion(bit);