88 lines
2.7 KiB
C
88 lines
2.7 KiB
C
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/* SPDX-License-Identifier: GPL-2.0 */
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/*
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* The owner field of the rw_semaphore structure will be set to
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* RWSEM_READER_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 4 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) RWSEM_ANONYMOUSLY_OWNED bit set with some other bits set as well
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* - lock is owned by an anonymous writer, so spinning on the lock
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* owner should be disabled.
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* 4) Other non-zero value
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* - a writer owns the lock and other writers can spin on the lock owner.
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*/
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#define RWSEM_ANONYMOUSLY_OWNED (1UL << 0)
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#define RWSEM_READER_OWNED ((struct task_struct *)RWSEM_ANONYMOUSLY_OWNED)
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#ifdef CONFIG_DEBUG_RWSEMS
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# define DEBUG_RWSEMS_WARN_ON(c) DEBUG_LOCKS_WARN_ON(c)
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#else
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# define DEBUG_RWSEMS_WARN_ON(c)
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#endif
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#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
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/*
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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,
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* may not need READ_ONCE() as long as the pointer value is only used
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* for comparison and isn't being dereferenced.
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*/
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static inline void rwsem_set_owner(struct rw_semaphore *sem)
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{
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WRITE_ONCE(sem->owner, current);
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}
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static inline void rwsem_clear_owner(struct rw_semaphore *sem)
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{
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WRITE_ONCE(sem->owner, NULL);
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}
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static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
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{
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/*
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* We check the owner value first to make sure that we will only
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* do a write to the rwsem cacheline when it is really necessary
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* to minimize cacheline contention.
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*/
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if (READ_ONCE(sem->owner) != RWSEM_READER_OWNED)
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WRITE_ONCE(sem->owner, RWSEM_READER_OWNED);
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}
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/*
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* Return true if the a rwsem waiter can spin on the rwsem's owner
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* and steal the lock, i.e. the lock is not anonymously owned.
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* N.B. !owner is considered spinnable.
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*/
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static inline bool is_rwsem_owner_spinnable(struct task_struct *owner)
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{
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return !((unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED);
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}
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/*
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* Return true if rwsem is owned by an anonymous writer or readers.
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*/
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static inline bool rwsem_has_anonymous_owner(struct task_struct *owner)
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{
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return (unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED;
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}
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#else
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static inline void rwsem_set_owner(struct rw_semaphore *sem)
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{
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}
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static inline void rwsem_clear_owner(struct rw_semaphore *sem)
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{
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}
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static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
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{
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}
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#endif
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