1 files changed, 28 insertions, 22 deletions

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 1f7734949ac8..b4233dc2c2b0 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -11,6 +11,13 @@
  * called from interrupt context and we have preemption disabled while
  * spinning.
  */
+
+struct optimistic_spin_node {
+	struct optimistic_spin_node *next, *prev;
+	int locked; /* 1 if lock acquired */
+	int cpu; /* encoded CPU # + 1 value */
+};
+
 static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);
 
 /*
@@ -37,32 +44,28 @@ static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
 /*
  * Get a stable @node->next pointer, either for unlock() or unqueue() purposes.
  * Can return NULL in case we were the last queued and we updated @lock instead.
+ *
+ * If osq_lock() is being cancelled there must be a previous node
+ * and 'old_cpu' is its CPU #.
+ * For osq_unlock() there is never a previous node and old_cpu is
+ * set to OSQ_UNLOCKED_VAL.
  */
 static inline struct optimistic_spin_node *
 osq_wait_next(struct optimistic_spin_queue *lock,
 	      struct optimistic_spin_node *node,
-	      struct optimistic_spin_node *prev)
+	      int old_cpu)
 {
-	struct optimistic_spin_node *next = NULL;
 	int curr = encode_cpu(smp_processor_id());
-	int old;
-
-	/*
-	 * If there is a prev node in queue, then the 'old' value will be
-	 * the prev node's CPU #, else it's set to OSQ_UNLOCKED_VAL since if
-	 * we're currently last in queue, then the queue will then become empty.
-	 */
-	old = prev ? prev->cpu : OSQ_UNLOCKED_VAL;
 
 	for (;;) {
 		if (atomic_read(&lock->tail) == curr &&
-		    atomic_cmpxchg_acquire(&lock->tail, curr, old) == curr) {
+		    atomic_cmpxchg_acquire(&lock->tail, curr, old_cpu) == curr) {
 			/*
 			 * We were the last queued, we moved @lock back. @prev
 			 * will now observe @lock and will complete its
 			 * unlock()/unqueue().
 			 */
-			break;
+			return NULL;
 		}
 
 		/*
@@ -76,15 +79,15 @@ osq_wait_next(struct optimistic_spin_queue *lock,
 		 * wait for a new @node->next from its Step-C.
 		 */
 		if (node->next) {
+			struct optimistic_spin_node *next;
+
 			next = xchg(&node->next, NULL);
 			if (next)
-				break;
+				return next;
 		}
 
 		cpu_relax();
 	}
-
-	return next;
 }
 
 bool osq_lock(struct optimistic_spin_queue *lock)
@@ -135,7 +138,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 	 */
 
 	/*
-	 * Wait to acquire the lock or cancelation. Note that need_resched()
+	 * Wait to acquire the lock or cancellation. Note that need_resched()
 	 * will come with an IPI, which will wake smp_cond_load_relaxed() if it
 	 * is implemented with a monitor-wait. vcpu_is_preempted() relies on
 	 * polling, be careful.
@@ -154,13 +157,17 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 	 */
 
 	for (;;) {
-		if (prev->next == node &&
+		/*
+		 * cpu_relax() below implies a compiler barrier which would
+		 * prevent this comparison being optimized away.
+		 */
+		if (data_race(prev->next) == node &&
 		    cmpxchg(&prev->next, node, NULL) == node)
 			break;
 
 		/*
 		 * We can only fail the cmpxchg() racing against an unlock(),
-		 * in which case we should observe @node->locked becomming
+		 * in which case we should observe @node->locked becoming
 		 * true.
 		 */
 		if (smp_load_acquire(&node->locked))
@@ -182,7 +189,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 	 * back to @prev.
 	 */
 
-	next = osq_wait_next(lock, node, prev);
+	next = osq_wait_next(lock, node, prev->cpu);
 	if (!next)
 		return false;
 
@@ -208,8 +215,7 @@ void osq_unlock(struct optimistic_spin_queue *lock)
 	/*
 	 * Fast path for the uncontended case.
 	 */
-	if (likely(atomic_cmpxchg_release(&lock->tail, curr,
-					  OSQ_UNLOCKED_VAL) == curr))
+	if (atomic_try_cmpxchg_release(&lock->tail, &curr, OSQ_UNLOCKED_VAL))
 		return;
 
 	/*
@@ -222,7 +228,7 @@ void osq_unlock(struct optimistic_spin_queue *lock)
 		return;
 	}
 
-	next = osq_wait_next(lock, node, NULL);
+	next = osq_wait_next(lock, node, OSQ_UNLOCKED_VAL);
 	if (next)
 		WRITE_ONCE(next->locked, 1);
 }