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 <pe...@hurleysoftware.com>
Signed-off-by: Waiman Long <waiman.l...@hpe.com>
Reviewed-by: Peter Hurley <pe...@hurleysoftware.com>
---
kernel/locking/rwsem-xadd.c | 38 ++++++++++++++++++++------------------
1 files changed, 20 insertions(+), 18 deletions(-)
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index e3a7e06..a85a2bd 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -332,9 +332,16 @@ 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) {
/*
@@ -354,7 +361,7 @@ 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.
@@ -364,7 +371,6 @@ 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();
@@ -376,21 +382,17 @@ 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;
@@ -402,7 +404,7 @@ 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;
/*
--
1.7.1
