[PATCH 04/11] qspinlock: Extract out the exchange of tail code word

Peter Zijlstra
2014-06-15T12:47:01Z

From: Waiman Long <Waiman.Long at hp.com>

This patch extracts the logic for the exchange of new and previous tail
code words into a new xchg_tail() function which can be optimized in a
later patch.

Signed-off-by: Waiman Long <Waiman.Long at hp.com>
Signed-off-by: Peter Zijlstra <peterz at infradead.org>
---
 include/asm-generic/qspinlock_types.h |    2 +
 kernel/locking/qspinlock.c            |   58 +++++++++++++++++++++-------------
 2 files changed, 38 insertions(+), 22 deletions(-)

--- a/include/asm-generic/qspinlock_types.h
+++ b/include/asm-generic/qspinlock_types.h
@@ -61,6 +61,8 @@ typedef struct qspinlock {
 #define _Q_TAIL_CPU_BITS	(32 - _Q_TAIL_CPU_OFFSET)
 #define _Q_TAIL_CPU_MASK	_Q_SET_MASK(TAIL_CPU)

+#define _Q_TAIL_MASK		(_Q_TAIL_IDX_MASK | _Q_TAIL_CPU_MASK)
+
 #define _Q_LOCKED_VAL		(1U << _Q_LOCKED_OFFSET)
 #define _Q_PENDING_VAL		(1U << _Q_PENDING_OFFSET)

--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -86,6 +86,31 @@ static inline struct mcs_spinlock *decod
 #define _Q_LOCKED_PENDING_MASK	(_Q_LOCKED_MASK | _Q_PENDING_MASK)

 /**
+ * xchg_tail - Put in the new queue tail code word & retrieve previous one
+ * @lock : Pointer to queue spinlock structure
+ * @tail : The new queue tail code word
+ * Return: The previous queue tail code word
+ *
+ * xchg(lock, tail)
+ *
+ * p,*,* -> n,*,* ; prev = xchg(lock, node)
+ */
+static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
+{
+	u32 old, new, val = atomic_read(&lock->val);
+
+	for (;;) {
+		new = (val & _Q_LOCKED_PENDING_MASK) | tail;
+		old = atomic_cmpxchg(&lock->val, val, new);
+		if (old == val)
+			break;
+
+		val = old;
+	}
+	return old;
+}
+
+/**
  * queue_spin_lock_slowpath - acquire the queue spinlock
  * @lock: Pointer to queue spinlock structure
  * @val: Current value of the queue spinlock 32-bit word
@@ -182,36 +207,25 @@ void queue_spin_lock_slowpath(struct qsp
 	node->next = NULL;

 	/*
-	 * we already touched the queueing cacheline; don't bother with pending
-	 * stuff.
-	 *
-	 * trylock || xchg(lock, node)
-	 *
-	 * 0,0,0 -> 0,0,1 ; trylock
-	 * p,y,x -> n,y,x ; prev = xchg(lock, node)
+	 * We touched a (possibly) cold cacheline in the per-cpu queue node;
+	 * attempt the trylock once more in the hope someone let go while we
+	 * weren't watching.
 	 */
-	for (;;) {
-		new = _Q_LOCKED_VAL;
-		if (val)
-			new = tail | (val & _Q_LOCKED_PENDING_MASK);
-
-		old = atomic_cmpxchg(&lock->val, val, new);
-		if (old == val)
-			break;
-
-		val = old;
-	}
+	if (queue_spin_trylock(lock))
+		goto release;

 	/*
-	 * we won the trylock; forget about queueing.
+	 * we already touched the queueing cacheline; don't bother with pending
+	 * stuff.
+	 *
+	 * p,*,* -> n,*,*
 	 */
-	if (new == _Q_LOCKED_VAL)
-		goto release;
+	old = xchg_tail(lock, tail);

 	/*
 	 * if there was a previous node; link it and wait.
 	 */
-	if (old & ~_Q_LOCKED_PENDING_MASK) {
+	if (old & _Q_TAIL_MASK) {
 		prev = decode_tail(old);
 		ACCESS_ONCE(prev->next) = node;