[Xen-devel] [PATCH v15 04/15] qspinlock: Extract out code snippets for the next patch

[Waiman Long <Waiman.Long@xxxxxx>]

This is a preparatory patch that extracts out the following 2 code
snippets to prepare for the next performance optimization patch.

 1) the logic for the exchange of new and previous tail code words
    into a new xchg_tail() function.
 2) the logic for clearing the pending bit and setting the locked bit
    into a new clear_pending_set_locked() function.

This patch also simplifies the trylock operation before queuing by
calling queue_spin_trylock() directly.

Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 include/asm-generic/qspinlock_types.h |    2 +
 kernel/locking/qspinlock.c            |   79 ++++++++++++++++++++-------------
 2 files changed, 50 insertions(+), 31 deletions(-)

diff --git a/include/asm-generic/qspinlock_types.h 
b/include/asm-generic/qspinlock_types.h
index 9c3f5c2..ef36613 100644
--- a/include/asm-generic/qspinlock_types.h
+++ b/include/asm-generic/qspinlock_types.h
@@ -58,6 +58,8 @@ 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)
 
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 0351f78..11f6ad9 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -97,6 +97,42 @@ static inline struct mcs_spinlock *decode_tail(u32 tail)
 #define _Q_LOCKED_PENDING_MASK (_Q_LOCKED_MASK | _Q_PENDING_MASK)
 
 /**
+ * clear_pending_set_locked - take ownership and clear the pending bit.
+ * @lock: Pointer to queue spinlock structure
+ *
+ * *,1,0 -> *,0,1
+ */
+static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
+{
+       atomic_add(-_Q_PENDING_VAL + _Q_LOCKED_VAL, &lock->val);
+}
+
+/**
+ * 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
@@ -178,15 +214,7 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 
val)
         *
         * *,1,0 -> *,0,1
         */
-       for (;;) {
-               new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL;
-
-               old = atomic_cmpxchg(&lock->val, val, new);
-               if (old == val)
-                       break;
-
-               val = old;
-       }
+       clear_pending_set_locked(lock);
        return;
 
        /*
@@ -203,37 +231,26 @@ queue:
        node->next = NULL;
 
        /*
-        * We have already touched the queueing cacheline; don't bother with
-        * pending stuff.
-        *
-        * trylock || xchg(lock, node)
-        *
-        * 0,0,0 -> 0,0,1 ; no tail, not locked -> no tail, locked.
-        * p,y,x -> n,y,x ; tail was p -> tail is n; preserving locked.
+        * 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 have 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 until reaching the
         * head of the waitqueue.
         */
-       if (old & ~_Q_LOCKED_PENDING_MASK) {
+       if (old & _Q_TAIL_MASK) {
                prev = decode_tail(old);
                WRITE_ONCE(prev->next, node);
 
-- 
1.7.1


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