locking/rwsem: Streamline the rwsem_optimistic_spin() code

This patch moves the owner loading and checking code entirely inside of
rwsem_spin_on_owner() to simplify the logic of rwsem_optimistic_spin()
loop.

Suggested-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Douglas Hatch <doug.hatch@hpe.com>
Cc: Jason Low <jason.low2@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Scott J Norton <scott.norton@hpe.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1463534783-38814-6-git-send-email-Waiman.Long@hpe.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 20 insertions, 18 deletions

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 4f1daf5a472d..2031281bb940 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -350,9 +350,16 @@ done:
         return ret;
 }
 
-static noinline
-bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
+/*
+ * Return true only if we can still spin on the owner field of the rwsem.
+ */
+static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
 {
+        struct task_struct *owner = READ_ONCE(sem->owner);
+
+        if (!rwsem_owner_is_writer(owner))
+                goto out;
+
         rcu_read_lock();
         while (sem->owner == owner) {
                 /*
@@ -372,7 +379,7 @@ bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
                 cpu_relax_lowlatency();
         }
         rcu_read_unlock();
-
+out:
         /*
          * If there is a new owner or the owner is not set, we continue
          * spinning.
@@ -382,7 +389,6 @@ bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
 
 static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
 {
-        struct task_struct *owner;
         bool taken = false;
 
         preempt_disable();
@@ -394,21 +400,17 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
         if (!osq_lock(&sem->osq))
                 goto done;
 
-        while (true) {
-                owner = READ_ONCE(sem->owner);
+        /*
+         * Optimistically spin on the owner field and attempt to acquire the
+         * lock whenever the owner changes. Spinning will be stopped when:
+         *  1) the owning writer isn't running; or
+         *  2) readers own the lock as we can't determine if they are
+         *     actively running or not.
+         */
+        while (rwsem_spin_on_owner(sem)) {
                 /*
-                 * Don't spin if
-                 * 1) the owner is a reader as we we can't determine if the
-                 *    reader is actively running or not.
-                 * 2) The rwsem_spin_on_owner() returns false which means
-                 *    the owner isn't running.
+                 * Try to acquire the lock
                  */
-                if (rwsem_owner_is_reader(owner) ||
-                   (rwsem_owner_is_writer(owner) &&
-                   !rwsem_spin_on_owner(sem, owner)))
-                        break;
-
-                /* wait_lock will be acquired if write_lock is obtained */
                 if (rwsem_try_write_lock_unqueued(sem)) {
                         taken = true;
                         break;
@@ -420,7 +422,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
                  * we're an RT task that will live-lock because we won't let
                  * the owner complete.
                  */
-                if (!owner && (need_resched() || rt_task(current)))
+                if (!sem->owner && (need_resched() || rt_task(current)))
                         break;
 
                 /*