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Subject[PATCH] locking/qspinlock: Add bug check for exceeding MAX_NODES
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 <>
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);

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