[PATCH] locking/qspinlock: Add bug check for exceeding MAX_NODES

[Waiman Long <longman@redhat.com>]

On some architectures, it is possible to have nested NMIs taking
spinlocks nestedly. Even though the chance of having more than 4 nested
spinlocks with contention is extremely small, there could still be a
possibility that it may happen some days leading to system panic.

What we don't want is a silent corruption with system panic somewhere
else. So add a BUG_ON() check to make sure that a system panic caused
by this will show the correct root cause.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/locking/qspinlock.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 8a8c3c2..f823221 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -412,6 +412,16 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 	idx = node->count++;
 	tail = encode_tail(smp_processor_id(), idx);
 
+	/*
+	 * 4 nodes are allocated based on the assumption that there will
+	 * not be nested NMIs taking spinlocks. That may not be true in
+	 * some architectures even though the chance of needing more than
+	 * 4 nodes will still be extremely unlikely. Adding a bug check
+	 * here to make sure there won't be a silent corruption in case
+	 * this condition happens.
+	 */
+	BUG_ON(idx >= MAX_NODES);
+
 	node = grab_mcs_node(node, idx);
 
 	/*
-- 
1.8.3.1