rcu: Inform the user about extended quiescent state on PROVE_RCU warning

[sfrench/cifs-2.6.git] / kernel / lockdep.c

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index b2e08c932d91c6507f07d637f61f0df1205778b8..f45c6817770e759cda7a954f82a6bd98f99a0ba9 100644 (file)
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -4170,6 +4170,28 @@ void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
        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);
+
+       /*
+        * If a CPU is in the RCU-free window in idle (ie: in the section
+        * between rcu_idle_enter() and rcu_idle_exit(), then RCU
+        * considers that CPU to be in an "extended quiescent state",
+        * which means that RCU will be completely ignoring that CPU.
+        * Therefore, rcu_read_lock() and friends have absolutely no
+        * effect on a CPU running in that state. In other words, even if
+        * such an RCU-idle CPU has called rcu_read_lock(), RCU might well
+        * delete data structures out from under it.  RCU really has no
+        * choice here: we need to keep an RCU-free window in idle where
+        * the CPU may possibly enter into low 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.
+        *
+        * So complain bitterly if someone does call rcu_read_lock(),
+        * rcu_read_lock_bh() and so on from extended quiescent states.
+        */
+       if (rcu_is_cpu_idle())
+               printk("RCU used illegally from extended quiescent state!\n");
+
        lockdep_print_held_locks(curr);
        printk("\nstack backtrace:\n");
        dump_stack();