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Subject[PATCH v4 13/16] locking/rwsem: Add more rwsem owner access helpers
Date
Before combining owner and count, we are adding two new helpers for
accessing the owner value in the rwsem.

1) struct task_struct *rwsem_get_owner(struct rw_semaphore *sem)
2) bool is_rwsem_reader_owned(struct rw_semaphore *sem)

Signed-off-by: Waiman Long <longman@redhat.com>
---
kernel/locking/rwsem.c | 50 +++++++++++++++++++++++++++++++-----------
1 file changed, 37 insertions(+), 13 deletions(-)

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 8b23009e6b2c..ab26aba65371 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -130,6 +130,11 @@ static inline void rwsem_clear_owner(struct rw_semaphore *sem)
WRITE_ONCE(sem->owner, NULL);
}

+static inline struct task_struct *rwsem_get_owner(struct rw_semaphore *sem)
+{
+ return READ_ONCE(sem->owner);
+}
+
/*
* The task_struct pointer of the last owning reader will be left in
* the owner field.
@@ -174,6 +179,23 @@ static inline bool is_rwsem_spinnable(struct rw_semaphore *sem)
return is_rwsem_owner_spinnable(READ_ONCE(sem->owner));
}

+/*
+ * Return true if the rwsem is owned by a reader.
+ */
+static inline bool is_rwsem_reader_owned(struct rw_semaphore *sem)
+{
+#ifdef CONFIG_DEBUG_RWSEMS
+ /*
+ * Check the count to see if it is write-locked.
+ */
+ long count = atomic_long_read(&sem->count);
+
+ if (count & RWSEM_WRITER_MASK)
+ return false;
+#endif
+ return (unsigned long)sem->owner & RWSEM_READER_OWNED;
+}
+
/*
* Return true if rwsem is owned by an anonymous writer or readers.
*/
@@ -193,6 +215,7 @@ static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
{
unsigned long val = (unsigned long)current | RWSEM_READER_OWNED
| RWSEM_ANONYMOUSLY_OWNED;
+
if (READ_ONCE(sem->owner) == (struct task_struct *)val)
cmpxchg_relaxed((unsigned long *)&sem->owner, val,
RWSEM_READER_OWNED | RWSEM_ANONYMOUSLY_OWNED);
@@ -530,7 +553,7 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)

preempt_disable();
rcu_read_lock();
- owner = READ_ONCE(sem->owner);
+ owner = rwsem_get_owner(sem);
if (owner) {
ret = is_rwsem_owner_spinnable(owner) &&
(is_rwsem_owner_reader(owner) || owner_on_cpu(owner));
@@ -562,15 +585,21 @@ enum owner_state {

static noinline enum owner_state rwsem_spin_on_owner(struct rw_semaphore *sem)
{
- struct task_struct *owner = READ_ONCE(sem->owner);
+ struct task_struct *owner = rwsem_get_owner(sem);
long count;

if (!is_rwsem_owner_spinnable(owner))
return OWNER_NONSPINNABLE;

rcu_read_lock();
- while (owner && !is_rwsem_owner_reader(owner)
- && (READ_ONCE(sem->owner) == owner)) {
+ while (owner && !is_rwsem_owner_reader(owner)) {
+ struct task_struct *new_owner = rwsem_get_owner(sem);
+
+ if (new_owner != owner) {
+ owner = new_owner;
+ break; /* The owner has changed */
+ }
+
/*
* Ensure we emit the owner->on_cpu, dereference _after_
* checking sem->owner still matches owner, if that fails,
@@ -597,7 +626,6 @@ static noinline enum owner_state rwsem_spin_on_owner(struct rw_semaphore *sem)
* spinning except when here is no active locks and the handoff bit
* is set. In this case, we have to stop spinning.
*/
- owner = READ_ONCE(sem->owner);
if (!is_rwsem_owner_spinnable(owner))
return OWNER_NONSPINNABLE;
if (owner && !is_rwsem_owner_reader(owner))
@@ -1093,8 +1121,7 @@ inline void __down_read(struct rw_semaphore *sem)
if (unlikely(atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
&sem->count) & RWSEM_READ_FAILED_MASK)) {
rwsem_down_read_failed(sem);
- DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner &
- RWSEM_READER_OWNED), sem);
+ DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
} else {
rwsem_set_reader_owned(sem);
}
@@ -1106,8 +1133,7 @@ static inline int __down_read_killable(struct rw_semaphore *sem)
&sem->count) & RWSEM_READ_FAILED_MASK)) {
if (IS_ERR(rwsem_down_read_failed_killable(sem)))
return -EINTR;
- DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner &
- RWSEM_READER_OWNED), sem);
+ DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
} else {
rwsem_set_reader_owned(sem);
}
@@ -1174,8 +1200,7 @@ inline void __up_read(struct rw_semaphore *sem)
{
long tmp;

- DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner & RWSEM_READER_OWNED),
- sem);
+ DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
rwsem_clear_reader_owned(sem);
tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS))
@@ -1382,8 +1407,7 @@ EXPORT_SYMBOL(down_write_killable_nested);

void up_read_non_owner(struct rw_semaphore *sem)
{
- DEBUG_RWSEMS_WARN_ON(!((unsigned long)sem->owner & RWSEM_READER_OWNED),
- sem);
+ DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
__up_read(sem);
}
EXPORT_SYMBOL(up_read_non_owner);
--
2.18.1
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