/*
* The owner field of the rw_semaphore structure will be set to
* RWSEM_READ_OWNED when a reader grabs the lock. A writer will clear
* the owner field when it unlocks. A reader, on the other hand, will
* not touch the owner field when it unlocks.
*
* In essence, the owner field now has the following 3 states:
* 1) 0
* - lock is free or the owner hasn't set the field yet
* 2) RWSEM_READER_OWNED
* - lock is currently or previously owned by readers (lock is free
* or not set by owner yet)
* 3) Other non-zero value
* - a writer owns the lock
*/
#define RWSEM_READER_OWNED ((struct task_struct *)1UL)
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
/*
* All writes to owner are protected by WRITE_ONCE() to make sure that
* store tearing can't happen as optimistic spinners may read and use
* the owner value concurrently without lock. Read from owner, however,
* may not need READ_ONCE() as long as the pointer value is only used
* for comparison and isn't being dereferenced.
*/
static inline void rwsem_set_owner(struct rw_semaphore *sem)
{
WRITE_ONCE(sem->owner, current);
}
static inline void rwsem_clear_owner(struct rw_semaphore *sem)
{
WRITE_ONCE(sem->owner, NULL);
}
static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
{
/*
* We check the owner value first to make sure that we will only
* do a write to the rwsem cacheline when it is really necessary
* to minimize cacheline contention.
*/
if (sem->owner != RWSEM_READER_OWNED)
WRITE_ONCE(sem->owner, RWSEM_READER_OWNED);
}
static inline bool rwsem_owner_is_writer(struct task_struct *owner)
{
return owner && owner != RWSEM_READER_OWNED;
}
static inline bool rwsem_owner_is_reader(struct task_struct *owner)
{
return owner == RWSEM_READER_OWNED;
}
#else
static inline void rwsem_set_owner(struct rw_semaphore *sem)
{
}
static inline void rwsem_clear_owner(struct rw_semaphore *sem)
{
}
static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
{
}
#endif
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