powerpc/qspinlock: Propagate sleepy if previous waiter is preempted

The sleepy (aka lock-owner-is-preempted) condition is propagated down
the queue by each waiter. If a waiter becomes preempted, it can no
longer propagate sleepy. To allow subsequent waiters to yield to the
lock owner, also check the lock owner in this case.

Signed-off-by: Nicholas Piggin <[email protected]>
Tested-by: Shrikanth Hegde <[email protected]>
Reviewed-by: "Nysal Jan K.A" <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
Link: https://msgid.link/[email protected]

Author: npiggin

arch/powerpc/lib/qspinlock.c

@@ -384,7 +384,11 @@ static __always_inline bool yield_to_prev(struct qspinlock *lock, struct qnode *
 	if (!pv_yield_propagate_owner)
 		goto yield_prev;
 
-	if (node->sleepy) {
+	/*
+	 * If the previous waiter was preempted it might not be able to
+	 * propagate sleepy to us, so check the lock in that case too.
+	 */
+	if (node->sleepy || vcpu_is_preempted(prev_cpu)) {
 		u32 val = READ_ONCE(lock->val);
 
 		if (val & _Q_LOCKED_VAL) {