[Xen-devel] [PATCH v10 09/19] qspinlock: Prepare for unfair lock support

[Waiman Long <Waiman.Long@xxxxxx>]

If unfair lock is supported, the lock acquisition loop at the end of
the queue_spin_lock_slowpath() function may need to detect the fact
the lock can be stolen. Code are added for the stolen lock detection.

A new qhead macro is also defined as a shorthand for mcs.locked.

Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
---
 kernel/locking/qspinlock.c |   26 +++++++++++++++++++-------
 1 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index e98d7d4..9e7659e 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -64,6 +64,7 @@
 struct qnode {
        struct mcs_spinlock mcs;
 };
+#define qhead  mcs.locked      /* The queue head flag */
 
 /*
  * Per-CPU queue node structures; we can never have more than 4 nested
@@ -216,18 +217,20 @@ xchg_tail(struct qspinlock *lock, u32 tail, u32 *pval)
 
 /**
  * get_qlock - Set the lock bit and own the lock
- * @lock: Pointer to queue spinlock structure
+ * @lock : Pointer to queue spinlock structure
+ * Return: 1 if lock acquired, 0 otherwise
  *
  * This routine should only be called when the caller is the only one
  * entitled to acquire the lock.
  */
-static __always_inline void get_qlock(struct qspinlock *lock)
+static __always_inline int get_qlock(struct qspinlock *lock)
 {
        struct __qspinlock *l = (void *)lock;
 
        barrier();
        ACCESS_ONCE(l->locked) = _Q_LOCKED_VAL;
        barrier();
+       return 1;
 }
 
 /**
@@ -365,7 +368,7 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 
val)
        tail = encode_tail(smp_processor_id(), idx);
 
        node += idx;
-       node->mcs.locked = 0;
+       node->qhead = 0;
        node->mcs.next = NULL;
 
        /*
@@ -391,7 +394,7 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 
val)
                prev = decode_tail(old);
                ACCESS_ONCE(prev->mcs.next) = (struct mcs_spinlock *)node;
 
-               while (!smp_load_acquire(&node->mcs.locked))
+               while (!smp_load_acquire(&node->qhead))
                        arch_mutex_cpu_relax();
        }
 
@@ -403,6 +406,7 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 
val)
         *
         * *,x,y -> *,0,0
         */
+retry_queue_wait:
        while ((val = smp_load_acquire(&lock->val.counter))
                                       & _Q_LOCKED_PENDING_MASK)
                arch_mutex_cpu_relax();
@@ -419,12 +423,20 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 
val)
         */
        for (;;) {
                if (val != tail) {
-                       get_qlock(lock);
-                       break;
+                       /*
+                        * The get_qlock function will only failed if the
+                        * lock was stolen.
+                        */
+                       if (get_qlock(lock))
+                               break;
+                       else
+                               goto retry_queue_wait;
                }
                old = atomic_cmpxchg(&lock->val, val, _Q_LOCKED_VAL);
                if (old == val)
                        goto release;   /* No contention */
+               else if (old & _Q_LOCKED_MASK)
+                       goto retry_queue_wait;
 
                val = old;
        }
@@ -435,7 +447,7 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 
val)
        while (!(next = (struct qnode *)ACCESS_ONCE(node->mcs.next)))
                arch_mutex_cpu_relax();
 
-       arch_mcs_spin_unlock_contended(&next->mcs.locked);
+       arch_mcs_spin_unlock_contended(&next->qhead);
 
 release:
        /*
-- 
1.7.1


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