rcu: Check for illegal use of RCU from offlined CPUs

Although it is legal to use RCU during early boot, it is anything
but legal to use RCU at runtime from an offlined CPU.  After all, RCU
explicitly ignores offlined CPUs.  This commit therefore adds checks
for runtime use of RCU from offlined CPUs.

These checks are not perfect, in particular, they can be subverted
through use of things like rcu_dereference_raw().  Note that it is not
possible to put checks in rcu_read_lock() and friends due to the fact
that these primitives are used in code that might be used under either
RCU or lock-based protection, which means that checking rcu_read_lock()
gets you fat piles of false positives.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 7 insertions, 4 deletions

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index e1b005918bbb..9a323728e60c 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -99,15 +99,18 @@ long srcu_batches_completed(struct srcu_struct *sp);
  * power mode. This way we can notice an extended quiescent state to
  * other CPUs that started a grace period. Otherwise we would delay any
  * grace period as long as we run in the idle task.
+ *
+ * Similarly, we avoid claiming an SRCU read lock held if the current
+ * CPU is offline.
  */
 static inline int srcu_read_lock_held(struct srcu_struct *sp)
 {
-	if (rcu_is_cpu_idle())
-		return 0;
-
 	if (!debug_lockdep_rcu_enabled())
 		return 1;
-
+	if (rcu_is_cpu_idle())
+		return 0;
+	if (!rcu_lockdep_current_cpu_online())
+		return 0;
 	return lock_is_held(&sp->dep_map);
 }