On Wed, Jan 22, 2020 at 06:04:56PM +0100, Peter Zijlstra wrote:

+/*
+ * cna_splice_head -- splice the entire secondary queue onto the head of the
+ * primary queue.
+ */
+static cna_splice_head(struct qspinlock *lock, u32 val,
+		       struct mcs_spinlock *node, struct mcs_spinlock *next)
+{
+	struct mcs_spinlock *head_2nd, *tail_2nd;
+
+	tail_2nd = decode_tail(node->locked);
+	head_2nd = tail_2nd->next;
+
+	if (lock) {

That should be: if (!next) {

+		u32 new = ((struct cna_node *)tail_2nd)->encoded_tail | _Q_LOCKED_VAL;
+		if (!atomic_try_cmpxchg_relaxed(&lock->val, &val, new))
+			return NULL;
+
+		/*
+		 * The moment we've linked the primary tail we can race with
+		 * the WRITE_ONCE(prev->next, node) store from new waiters.
+		 * That store must win.
+		 */

And that still is a shit comment; I'll go try again.

+		cmpxchg_relaxed(&tail_2nd->next, head_2nd, next);
+	} else {
+		tail_2nd->next = next;
+	}
+
+	return head_2nd;
+}

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