[PATCH v2] ftrace: make extra rcu_is_watching() validation check optional

[PATCH v2] ftrace: make extra rcu_is_watching() validation check optional

[Andrii Nakryiko]

Introduce CONFIG_FTRACE_VALIDATE_RCU_IS_WATCHING config option to
control whether ftrace low-level code performs additional
rcu_is_watching()-based validation logic in an attempt to catch noinstr
violations.

This check is expected to never be true and is mostly useful for
low-level debugging of ftrace subsystem. For most users it should
probably be kept disabled to eliminate unnecessary runtime overhead.

Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 include/linux/trace_recursion.h |  2 +-
 kernel/trace/Kconfig            | 14 ++++++++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
index d48cd92d2364..24ea8ac049b4 100644
--- a/include/linux/trace_recursion.h
+++ b/include/linux/trace_recursion.h
@@ -135,7 +135,7 @@ extern void ftrace_record_recursion(unsigned long ip, unsigned long parent_ip);
 # define do_ftrace_record_recursion(ip, pip)	do { } while (0)
 #endif
 
-#ifdef CONFIG_ARCH_WANTS_NO_INSTR
+#ifdef CONFIG_FTRACE_VALIDATE_RCU_IS_WATCHING
 # define trace_warn_on_no_rcu(ip)					\
 	({								\
 		bool __ret = !rcu_is_watching();			\
diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 61c541c36596..fcf45d5c60cb 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -974,6 +974,20 @@ config FTRACE_RECORD_RECURSION_SIZE
 	  This file can be reset, but the limit can not change in
 	  size at runtime.
 
+config FTRACE_VALIDATE_RCU_IS_WATCHING
+	bool "Validate RCU is on during ftrace recursion check"
+	depends on FUNCTION_TRACER
+	depends on ARCH_WANTS_NO_INSTR
+	help
+	  All callbacks that attach to the function tracing have some sort
+	  of protection against recursion. This option performs additional
+	  checks to make sure RCU is on when ftrace callbacks recurse.
+
+	  This is a feature useful for debugging ftrace. This will add more
+	  overhead to all ftrace-based invocations.
+
+	  If unsure, say N
+
 config RING_BUFFER_RECORD_RECURSION
 	bool "Record functions that recurse in the ring buffer"
 	depends on FTRACE_RECORD_RECURSION
-- 
2.43.0