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/* SPDX-License-Identifier: GPL-2.0 */
/*
* The owner field of the rw_semaphore structure will be set to
* RWSEM_READER_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 4 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) RWSEM_ANONYMOUSLY_OWNED bit set with some other bits set as well
* - lock is owned by an anonymous writer, so spinning on the lock
* owner should be disabled.
* 4) Other non-zero value
* - a writer owns the lock and other writers can spin on the lock owner.
*/
#define RWSEM_ANONYMOUSLY_OWNED (1UL << 0)
#define RWSEM_READER_OWNED ((struct task_struct *)RWSEM_ANONYMOUSLY_OWNED)
#ifdef CONFIG_DEBUG_RWSEMS
# define DEBUG_RWSEMS_WARN_ON(c) DEBUG_LOCKS_WARN_ON(c)
#else
# define DEBUG_RWSEMS_WARN_ON(c)
#endif
#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 (READ_ONCE(sem->owner) != RWSEM_READER_OWNED)
WRITE_ONCE(sem->owner, RWSEM_READER_OWNED);
}
/*
* Return true if the a rwsem waiter can spin on the rwsem's owner
* and steal the lock, i.e. the lock is not anonymously owned.
* N.B. !owner is considered spinnable.
*/
static inline bool is_rwsem_owner_spinnable(struct task_struct *owner)
{
return !((unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED);
}
/*
* Return true if rwsem is owned by an anonymous writer or readers.
*/
static inline bool rwsem_has_anonymous_owner(struct task_struct *owner)
{
return (unsigned long)owner & RWSEM_ANONYMOUSLY_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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