[PATCH 3/4] locking/qrwlock: Reduce writer to writer lock transfer latency

[Waiman Long <Waiman.Long@hp.com>]

In most cases, a writer acquires the lock in two steps - first setting
the writer mode byte to _QW_WAITING and then to _QW_LOCKED. So two
atomic operations are required. This 2-step dance is only needed if
readers are present. This patch modifies the logic so that a writer
will try to acquire the lock in a single step as long as possible
until it see some readers.

Using a locking microbenchmark, a 10-threads 5M locking loop of only
writers has the following performance numbers in a Haswell-EX box:

        Kernel          Locking Rate (Kops/s)
        ------          ---------------------
        4.1.1               11,939,648
        Patched 4.1.1       12,906,593

Signed-off-by: Waiman Long <Waiman.Long@hp.com>
---
 kernel/locking/qrwlock.c |   20 +++++++++++++-------
 1 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index ecd2d19..87e2d6b 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -109,15 +109,22 @@ EXPORT_SYMBOL(queue_read_lock_slowpath);
  */
 void queue_write_lock_slowpath(struct qrwlock *lock)
 {
-	u32 cnts;
-
 	/* Put the writer into the wait queue */
 	arch_spin_lock(&lock->lock);
 
 	/* Try to acquire the lock directly if no reader is present */
-	if (!atomic_read(&lock->cnts) &&
-	    (atomic_cmpxchg(&lock->cnts, 0, _QW_LOCKED) == 0))
-		goto unlock;
+	for (;;) {
+		u32 cnts = atomic_read(&lock->cnts);
+
+		if (!cnts) {
+			cnts = atomic_cmpxchg(&lock->cnts, 0, _QW_LOCKED);
+			if (cnts == 0)
+				goto unlock;
+		}
+		if (cnts & ~_QW_WMASK)
+			break;	/* Reader is present */
+		cpu_relax_lowlatency();
+	}
 
 	/*
 	 * Set the waiting flag to notify readers that a writer is pending,
@@ -135,8 +142,7 @@ void queue_write_lock_slowpath(struct qrwlock *lock)
 
 	/* When no more readers, set the locked flag */
 	for (;;) {
-		cnts = atomic_read(&lock->cnts);
-		if ((cnts == _QW_WAITING) &&
+		if ((atomic_read(&lock->cnts) == _QW_WAITING) &&
 		    (atomic_cmpxchg(&lock->cnts, _QW_WAITING,
 				    _QW_LOCKED) == _QW_WAITING))
 			break;
-- 
1.7.1