/*
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* The owner field of the rw_semaphore structure will be set to
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* RWSEM_READ_OWNED when a reader grabs the lock. A writer will clear
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* the owner field when it unlocks. A reader, on the other hand, will
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* not touch the owner field when it unlocks.
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*
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* In essence, the owner field now has the following 3 states:
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* 1) 0
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* - lock is free or the owner hasn't set the field yet
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* 2) RWSEM_READER_OWNED
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* - lock is currently or previously owned by readers (lock is free
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* or not set by owner yet)
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* 3) Other non-zero value
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* - a writer owns the lock
|
*/
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#define RWSEM_READER_OWNED ((struct task_struct *)1UL)
|
|
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
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/*
|
* All writes to owner are protected by WRITE_ONCE() to make sure that
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* store tearing can't happen as optimistic spinners may read and use
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* 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.
|
*/
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static inline void rwsem_set_owner(struct rw_semaphore *sem)
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{
|
WRITE_ONCE(sem->owner, current);
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}
|
|
static inline void rwsem_clear_owner(struct rw_semaphore *sem)
|
{
|
WRITE_ONCE(sem->owner, NULL);
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}
|
|
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);
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}
|
|
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;
|
}
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#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
|
