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Subject[PATCH-tip 18/22] locking/rwsem: Make rwsem_spin_on_owner() return a tri-state value
Date
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 <longman@redhat.com>
---
kernel/locking/rwsem-xadd.c | 25 +++++++++++++++++++------
kernel/locking/rwsem-xadd.h | 5 +++++
2 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 21d462f..0a29aac 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -305,14 +305,24 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
}

/*
- * 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 changes and no reader is detected yet.
+ * 0 when owner changes and the new owner may be a reader.
+ * -1 when optimistic spinning has to stop because either the owner stops
+ * running, is unknown, or its timeslice has been used up.
*/
-static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
+enum owner_state {
+ OWNER_SPINNABLE = 1,
+ OWNER_READER = 0,
+ OWNER_NONSPINNABLE = -1,
+};
+
+static noinline enum owner_state rwsem_spin_on_owner(struct rw_semaphore *sem)
{
struct task_struct *owner = rwsem_get_owner(sem);

if (!is_rwsem_owner_spinnable(owner))
- return false;
+ return OWNER_NONSPINNABLE;

rcu_read_lock();
/*
@@ -337,7 +347,7 @@ static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
*/
if (need_resched() || !owner_on_cpu(owner, sem)) {
rcu_read_unlock();
- return false;
+ return OWNER_NONSPINNABLE;
}

cpu_relax();
@@ -348,7 +358,10 @@ static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
* If there is a new owner or the owner is not set, we continue
* spinning.
*/
- return is_rwsem_owner_spinnable(rwsem_get_owner(sem));
+ owner = rwsem_get_owner(sem);
+ if (!is_rwsem_owner_spinnable(owner))
+ return OWNER_NONSPINNABLE;
+ return is_rwsem_owner_reader(owner) ? OWNER_READER : OWNER_SPINNABLE;
}

static bool rwsem_optimistic_spin(struct rw_semaphore *sem, const long wlock)
@@ -371,7 +384,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem, const long wlock)
* 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) == OWNER_SPINNABLE) {
/*
* Try to acquire the lock
*/
diff --git a/kernel/locking/rwsem-xadd.h b/kernel/locking/rwsem-xadd.h
index d54b5db..1de6f1e 100644
--- a/kernel/locking/rwsem-xadd.h
+++ b/kernel/locking/rwsem-xadd.h
@@ -200,6 +200,11 @@ static inline bool is_rwsem_owner_spinnable(struct task_struct *owner)
return !((unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED);
}

+static inline bool is_rwsem_owner_reader(struct task_struct *owner)
+{
+ return (unsigned long)owner & RWSEM_READER_OWNED;
+}
+
/*
* Return true if the rwsem is owned by a reader.
*/
--
1.8.3.1
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