[RFC PATCH-tip v4 03/10] locking/rwsem: Make rwsem_spin_on_owner() return a tri-state value

Waiman Long
2016-08-18T21:11:05Z

This patch modifies rwsem_spin_on_owner() to return a tri-state value
to better reflect the state of lock holder which enables us to make a
better decision of what to do next.

Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
---
kernel/locking/rwsem-xadd.c | 14 +++++++++-----
1 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 9309e72..31cb286 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -352,9 +352,13 @@ done:
}

/*
- * Return true only if we can still spin on the owner field of the rwsem.
+ * Return the folowing three values depending on the lock owner state.
+ * 1 when owner has changed and no reader is detected yet.
+ * 0 when owner has change and/or owner is a reader.
+ * -1 when optimistic spinning has to stop because either the owner stops
+ * running or its timeslice has been used up.
*/
-static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
+static noinline int rwsem_spin_on_owner(struct rw_semaphore *sem)
{
struct task_struct *owner = READ_ONCE(sem->owner);

@@ -374,7 +378,7 @@ static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
/* abort spinning when need_resched or owner is not running */
if (!owner->on_cpu || need_resched()) {
rcu_read_unlock();
- return false;
+ return -1;
}

cpu_relax_lowlatency();
@@ -385,7 +389,7 @@ out:
* If there is a new owner or the owner is not set, we continue
* spinning.
*/
- return !rwsem_owner_is_reader(READ_ONCE(sem->owner));
+ return rwsem_owner_is_reader(READ_ONCE(sem->owner)) ? 0 : 1;
}

static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
@@ -408,7 +412,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
* 2) readers own the lock as we can't determine if they are
* actively running or not.
*/
- while (rwsem_spin_on_owner(sem)) {
+ while (rwsem_spin_on_owner(sem) > 0) {
/*
* Try to acquire the lock
*/
--
1.7.1