lockdep: Add CPU-idle/offline warning to lockdep-RCU splat

It is illegal to use RCU from a CPU that has reported idleness or
offlinedness to RCU.  However, it can be quite difficult to determine
from a stack trace whether or not a given CPU is idle or offline.
Therefore, this commit adds idle/offline diagnostics to the lockdep-RCU
error message.

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, 1 deletions

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 8889f7dd7c46..ea9ee4518c35 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -4176,7 +4176,13 @@ void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
         printk("-------------------------------\n");
         printk("%s:%d %s!\n", file, line, s);
         printk("\nother info that might help us debug this:\n\n");
-        printk("\nrcu_scheduler_active = %d, debug_locks = %d\n", rcu_scheduler_active, debug_locks);
+        printk("\n%srcu_scheduler_active = %d, debug_locks = %d\n",
+               !rcu_lockdep_current_cpu_online()
+                        ? "RCU used illegally from offline CPU!\n"
+                        : rcu_is_cpu_idle()
+                                ? "RCU used illegally from idle CPU!\n"
+                                : "",
+               rcu_scheduler_active, debug_locks);
 
         /*
          * If a CPU is in the RCU-free window in idle (ie: in the section