ftrace: Add ftrace_test_recursion_trylock() helper function

To make it easier for ftrace callbacks to have recursion protection, provide
a ftrace_test_recursion_trylock() and ftrace_test_recursion_unlock() helper
that tests for recursion.

Link: https://lkml.kernel.org/r/20201028115612.634927593@goodmis.org
Link: https://lkml.kernel.org/r/20201106023546.378584067@goodmis.orgSigned-off-by: NSteven Rostedt (VMware) <rostedt@goodmis.org>

include/linux/trace_recursion.h

@@ -183,5 +183,30 @@ static __always_inline void trace_clear_recursion(int bit)
 	current->trace_recursion = val;
 }
 
+/**
+ * ftrace_test_recursion_trylock - tests for recursion in same context
+ *
+ * Use this for ftrace callbacks. This will detect if the function
+ * tracing recursed in the same context (normal vs interrupt),
+ *
+ * Returns: -1 if a recursion happened.
+ *           >= 0 if no recursion
+ */
+static __always_inline int ftrace_test_recursion_trylock(void)
+{
+	return trace_test_and_set_recursion(TRACE_FTRACE_START, TRACE_FTRACE_MAX);
+}
+
+/**
+ * ftrace_test_recursion_unlock - called when function callback is complete
+ * @bit: The return of a successful ftrace_test_recursion_trylock()
+ *
+ * This is used at the end of a ftrace callback.
+ */
+static __always_inline void ftrace_test_recursion_unlock(int bit)
+{
+	trace_clear_recursion(bit);
+}
+
 #endif /* CONFIG_TRACING */
 #endif /* _LINUX_TRACE_RECURSION_H */

kernel/trace/trace_functions.c

@@ -141,22 +141,20 @@ function_trace_call(unsigned long ip, unsigned long parent_ip,
 	if (unlikely(!tr->function_enabled))
 		return;
 
+	bit = ftrace_test_recursion_trylock();
+	if (bit < 0)
+		return;
+
 	pc = preempt_count();
 	preempt_disable_notrace();
 
-	bit = trace_test_and_set_recursion(TRACE_FTRACE_START, TRACE_FTRACE_MAX);
-	if (bit < 0)
-		goto out;
-
 	cpu = smp_processor_id();
 	data = per_cpu_ptr(tr->array_buffer.data, cpu);
 	if (!atomic_read(&data->disabled)) {
 		local_save_flags(flags);
 		trace_function(tr, ip, parent_ip, flags, pc);
 	}
-	trace_clear_recursion(bit);
-
- out:
+	ftrace_test_recursion_unlock(bit);
 	preempt_enable_notrace();
 }