[PATCH v6 4/6] locking/rwsem: Change waiter->hanodff_set to a handoff_state enum

From: Waiman Long
Date: Thu Nov 17 2022 - 21:22:29 EST


Change the boolean waiter->handoff_set to an enum type so that we can
have more states in some later patches. Also use READ_ONCE() outside
wait_lock critical sections for read and WRITE_ONCE() inside wait_lock
critical sections for write for proper synchronization. There is no
functional change.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
kernel/locking/rwsem.c | 21 +++++++++++++--------
1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 8159a69b5de8..aa771fa1a1fe 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -335,12 +335,17 @@ enum rwsem_waiter_type {
RWSEM_WAITING_FOR_READ
};

+enum rwsem_handoff_state {
+ HANDOFF_NONE = 0,
+ HANDOFF_REQUESTED,
+};
+
struct rwsem_waiter {
struct list_head list;
struct task_struct *task;
enum rwsem_waiter_type type;
+ enum rwsem_handoff_state handoff_state;
unsigned long timeout;
- bool handoff_set;
};
#define rwsem_first_waiter(sem) \
list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
@@ -467,7 +472,7 @@ static void rwsem_mark_wake(struct rw_semaphore *sem,
adjustment -= RWSEM_FLAG_HANDOFF;
lockevent_inc(rwsem_rlock_handoff);
}
- waiter->handoff_set = true;
+ WRITE_ONCE(waiter->handoff_state, HANDOFF_REQUESTED);
}

atomic_long_add(-adjustment, &sem->count);
@@ -619,7 +624,7 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
* waiter is the one that set it. Otherwisee, we
* still try to acquire the rwsem.
*/
- if (first->handoff_set && (waiter != first))
+ if (first->handoff_state && (waiter != first))
return false;
}

@@ -647,11 +652,11 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,

/*
* We have either acquired the lock with handoff bit cleared or set
- * the handoff bit. Only the first waiter can have its handoff_set
+ * the handoff bit. Only the first waiter can have its handoff_state
* set here to enable optimistic spinning in slowpath loop.
*/
if (new & RWSEM_FLAG_HANDOFF) {
- first->handoff_set = true;
+ WRITE_ONCE(first->handoff_state, HANDOFF_REQUESTED);
lockevent_inc(rwsem_wlock_handoff);
return false;
}
@@ -1035,7 +1040,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat
waiter.task = current;
waiter.type = RWSEM_WAITING_FOR_READ;
waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
- waiter.handoff_set = false;
+ waiter.handoff_state = HANDOFF_NONE;

raw_spin_lock_irq(&sem->wait_lock);
if (list_empty(&sem->wait_list)) {
@@ -1122,7 +1127,7 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
waiter.task = current;
waiter.type = RWSEM_WAITING_FOR_WRITE;
waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
- waiter.handoff_set = false;
+ waiter.handoff_state = HANDOFF_NONE;

raw_spin_lock_irq(&sem->wait_lock);
rwsem_add_waiter(sem, &waiter);
@@ -1167,7 +1172,7 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
* In this case, we attempt to acquire the lock again
* without sleeping.
*/
- if (waiter.handoff_set) {
+ if (READ_ONCE(waiter.handoff_state)) {
enum owner_state owner_state;

owner_state = rwsem_spin_on_owner(sem);
--
2.31.1