[PATCH] tracing: generate RCU warnings even when tracepoints are disabled

[Dave Hansen <dave@sr71.net>]

From: Dave Hansen <dave.hansen@linux.intel.com>

Dave Jones reported seeing a bug from one of my TLB tracepoints:

	http://lkml.kernel.org/r/20140806181801.GA4605@redhat.com

I've been running these patches for months and never saw this.
But, a big chunk of my testing, especially with all the debugging
enabled, was in a vm where intel_idle doesn't work.  On the
systems where I was using intel_idle, I never had lockdep enabled
and this tracepoint on at the same time.

This patch ensures that whenever we have lockdep available, we do
_some_ RCU activity at the site of the tracepoint, despite
whether the tracepoint's condition matches or even if the
tracepoint itself is completely disabled.  This is a bit of a
hack, but it is pretty self-contained.

I confirmed that with this patch plus lockdep I get the same
splat as Dave Jones did, but without enabling the tracepoint
explicitly.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dave Jones <davej@redhat.com>,
Cc: paulmck@linux.vnet.ibm.com
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@redhat.com>

---

 b/include/linux/tracepoint.h |   11 +++++++++++
 1 file changed, 11 insertions(+)

diff -puN include/linux/tracepoint.h~easier-rcu-splat include/linux/tracepoint.h
--- a/include/linux/tracepoint.h~easier-rcu-splat	2014-08-07 10:29:32.701217956 -0700
+++ b/include/linux/tracepoint.h	2014-08-07 10:40:35.244627014 -0700
@@ -157,6 +157,12 @@ extern void syscall_unregfunc(void);
  * Make sure the alignment of the structure in the __tracepoints section will
  * not add unwanted padding between the beginning of the section and the
  * structure. Force alignment to the same alignment as the section start.
+ *
+ * When lockdep is enabled, we make sure to always do the RCU portions of
+ * the tracepoint code, regardless of whether tracing is on or we match the
+ * condition.  This lets us find RCU issues triggered with tracepoints even
+ * when this tracepoint is off.  This code has no purpose other than poking
+ * RCU a bit.
  */
 #define __DECLARE_TRACE(name, proto, args, cond, data_proto, data_args) \
 	extern struct tracepoint __tracepoint_##name;			\
@@ -167,6 +173,11 @@ extern void syscall_unregfunc(void);
 				TP_PROTO(data_proto),			\
 				TP_ARGS(data_args),			\
 				TP_CONDITION(cond),,);			\
+		if (IS_ENABLED(CONFIG_LOCKDEP)) {			\
+			rcu_read_lock_sched_notrace();			\
+			rcu_dereference_sched(__tracepoint_##name.funcs);\
+			rcu_read_unlock_sched_notrace();		\
+		}							\
 	}								\
 	__DECLARE_TRACE_RCU(name, PARAMS(proto), PARAMS(args),		\
 		PARAMS(cond), PARAMS(data_proto), PARAMS(data_args))	\
_