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>

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 4f1daf5..2031281 100644 (file)
--- 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;
 
                /*