rcu: Make RCU_LOCKDEP_WARN() avoid early lockdep checks

Currently, RCU_LOCKDEP_WARN() checks the condition before checking
to see if lockdep is still enabled.  This is necessary to avoid the
false-positive splats fixed by commit 3066820034 ("rcu: Reject
RCU_LOCKDEP_WARN() false positives").  However, the current state can
result in false-positive splats during early boot before lockdep is fully
initialized.  This commit therefore checks debug_lockdep_rcu_enabled()
both before and after checking the condition, thus avoiding both sets
of false-positive error reports.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Reported-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reported-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

include/linux/rcupdate.h

@@ -374,11 +374,18 @@ static inline int debug_lockdep_rcu_enabled(void)
  * RCU_LOCKDEP_WARN - emit lockdep splat if specified condition is met
  * @c: condition to check
  * @s: informative message
+ *
+ * This checks debug_lockdep_rcu_enabled() before checking (c) to
+ * prevent early boot splats due to lockdep not yet being initialized,
+ * and rechecks it after checking (c) to prevent false-positive splats
+ * due to races with lockdep being disabled.  See commit 3066820034b5dd
+ * ("rcu: Reject RCU_LOCKDEP_WARN() false positives") for more detail.
  */
 #define RCU_LOCKDEP_WARN(c, s)						\
 	do {								\
 		static bool __section(".data.unlikely") __warned;	\
-		if ((c) && debug_lockdep_rcu_enabled() && !__warned) {	\
+		if (debug_lockdep_rcu_enabled() && (c) &&		\
+		    debug_lockdep_rcu_enabled() && !__warned) {		\
 			__warned = true;				\
 			lockdep_rcu_suspicious(__FILE__, __LINE__, s);	\
 		}							\