locking/pvqspinlock: Relax cmpxchg's to improve performance on some archs

All the locking related cmpxchg's in the following functions are replaced with the _acquire variants:
- pv_queued_spin_steal_lock()
- trylock_clear_pending()

This change should help performance on architectures that use LL/SC.

On a 2-core 16-thread Power8 system with pvqspinlock explicitly enabled, the performance of a locking microbenchmark with and without this patch on a 4.10-rc8 kernel with Xinhui's PPC qspinlock patch were as follows:

  # of thread     w/o patch    with patch      % Change
  -----------     ---------    ----------      --------
       4         4053.3 Mop/s  4223.7 Mop/s     +4.2%
       8         3310.4 Mop/s  3406.0 Mop/s     +2.9%
      12         2576.4 Mop/s  2674.6 Mop/s     +3.8%

Signed-off-by Waiman Long <[email protected]>

---

v4->v5:
- Correct some grammatical issues in comment.

v3->v4:
- Update the comment in pv_kick_node() to mention that the code may not work in some archs.

v2->v3:
- Reduce scope by relaxing cmpxchg's in fast path only.

v1->v2:
- Add comments in changelog and code for the rationale of the change.

 kernel/locking/qspinlock_paravirt.h | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

kernel/locking/qspinlock_paravirt.h +13 -6

index e6b2f7a..4614e39 100644
@@ -72,7 +72,7 @@ static inline bool pv_queued_spin_steal_lock(struct qspinlock *lock)
 	struct __qspinlock *l = (void *)lock;
 
 	if (!(atomic_read(&lock->val) & _Q_LOCKED_PENDING_MASK) &&
-	    (cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0)) {
+	    (cmpxchg_acquire(&l->locked, 0, _Q_LOCKED_VAL) == 0)) {
 		qstat_inc(qstat_pv_lock_stealing, true);
 		return true;
 	}

@@ -101,16 +101,16 @@ static __always_inline void clear_pending(struct qspinlock *lock)
 
 /*
  * The pending bit check in pv_queued_spin_steal_lock() isn't a memory
- * barrier. Therefore, an atomic cmpxchg() is used to acquire the lock
- * just to be sure that it will get it.
+ * barrier. Therefore, an atomic cmpxchg_acquire() is used to acquire the
+ * lock just to be sure that it will get it.
  */
 static __always_inline int trylock_clear_pending(struct qspinlock *lock)
 {
 	struct __qspinlock *l = (void *)lock;
 
 	return !READ_ONCE(l->locked) &&
-	       (cmpxchg(&l->locked_pending, _Q_PENDING_VAL, _Q_LOCKED_VAL)
-			== _Q_PENDING_VAL);
+	       (cmpxchg_acquire(&l->locked_pending, _Q_PENDING_VAL,
+				_Q_LOCKED_VAL) == _Q_PENDING_VAL);
 }
 #else /* _Q_PENDING_BITS == 8 */
 static __always_inline void set_pending(struct qspinlock *lock)

@@ -138,7 +138,7 @@ static __always_inline int trylock_clear_pending(struct qspinlock *lock)
 		 */
 		old = val;
 		new = (val & ~_Q_PENDING_MASK) | _Q_LOCKED_VAL;
-		val = atomic_cmpxchg(&lock->val, old, new);
+		val = atomic_cmpxchg_acquire(&lock->val, old, new);
 
 		if (val == old)
 			return 1;

@@ -361,6 +361,13 @@ static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node)
 	 * observe its next->locked value and advance itself.
 	 *
 	 * Matches with smp_store_mb() and cmpxchg() in pv_wait_node()
+	 *
+	 * The write to next->locked in arch_mcs_spin_unlock_contended()
+	 * must be ordered before the read of pn->state in the cmpxchg()
+	 * below for the code to work correctly. However, this is not
+	 * guaranteed on all architectures when the cmpxchg() call fails.
+	 * Both x86 and PPC can provide that guarantee, but other
+	 * architectures not necessarily.
 	 */
 	if (cmpxchg(&pn->state, vcpu_halted, vcpu_hashed) != vcpu_halted)
 		return;