rcu: Improve diagnostics for blocked critical sections in irq

If an RCU read-side critical section occurs within an interrupt handler
or a softirq handler, it cannot have been preempted.  Therefore, there is
a check in rcu_read_unlock_special() checking for this error.  However,
when this check triggers, it lacks diagnostic information.  This commit
therefore moves rcu_read_unlock()'s lockdep annotation to follow the
call to __rcu_read_unlock() and changes rcu_read_unlock_special()'s
WARN_ON_ONCE() to an lockdep_rcu_suspicious() in order to locate where
the offending RCU read-side critical section began.  In addition, the
value of the ->rcu_read_unlock_special field is printed.

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

1 files changed, 7 insertions, 1 deletions

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 0a571e9a0f1d..8a33920b8845 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -334,7 +334,13 @@ void rcu_read_unlock_special(struct task_struct *t)
 	}
 
 	/* Hardware IRQ handlers cannot block, complain if they get here. */
-	if (WARN_ON_ONCE(in_irq() || in_serving_softirq())) {
+	if (in_irq() || in_serving_softirq()) {
+		lockdep_rcu_suspicious(__FILE__, __LINE__,
+				       "rcu_read_unlock() from irq or softirq with blocking in critical section!!!\n");
+		pr_alert("->rcu_read_unlock_special: %#x (b: %d, nq: %d)\n",
+			 t->rcu_read_unlock_special.s,
+			 t->rcu_read_unlock_special.b.blocked,
+			 t->rcu_read_unlock_special.b.need_qs);
 		local_irq_restore(flags);
 		return;
 	}