1 /* SPDX-License-Identifier: GPL-2.0 */
2 /*
3 * The owner field of the rw_semaphore structure will be set to
4 * RWSEM_READER_OWNED when a reader grabs the lock. A writer will clear
5 * the owner field when it unlocks. A reader, on the other hand, will
6 * not touch the owner field when it unlocks.
7 *
8 * In essence, the owner field now has the following 4 states:
9 * 1) 0
10 * - lock is free or the owner hasn't set the field yet
11 * 2) RWSEM_READER_OWNED
12 * - lock is currently or previously owned by readers (lock is free
13 * or not set by owner yet)
14 * 3) RWSEM_ANONYMOUSLY_OWNED bit set with some other bits set as well
15 * - lock is owned by an anonymous writer, so spinning on the lock
16 * owner should be disabled.
17 * 4) Other non-zero value
18 * - a writer owns the lock and other writers can spin on the lock owner.
19 */
20 #define RWSEM_ANONYMOUSLY_OWNED (1UL << 0)
21 #define RWSEM_READER_OWNED ((struct task_struct *)RWSEM_ANONYMOUSLY_OWNED)
22
23 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
24 /*
25 * All writes to owner are protected by WRITE_ONCE() to make sure that
26 * store tearing can't happen as optimistic spinners may read and use
27 * the owner value concurrently without lock. Read from owner, however,
28 * may not need READ_ONCE() as long as the pointer value is only used
29 * for comparison and isn't being dereferenced.
30 */
rwsem_set_owner(struct rw_semaphore * sem)31 static inline void rwsem_set_owner(struct rw_semaphore *sem)
32 {
33 WRITE_ONCE(sem->owner, current);
34 }
35
rwsem_clear_owner(struct rw_semaphore * sem)36 static inline void rwsem_clear_owner(struct rw_semaphore *sem)
37 {
38 WRITE_ONCE(sem->owner, NULL);
39 }
40
rwsem_set_reader_owned(struct rw_semaphore * sem)41 static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
42 {
43 /*
44 * We check the owner value first to make sure that we will only
45 * do a write to the rwsem cacheline when it is really necessary
46 * to minimize cacheline contention.
47 */
48 if (sem->owner != RWSEM_READER_OWNED)
49 WRITE_ONCE(sem->owner, RWSEM_READER_OWNED);
50 }
51
52 /*
53 * Return true if the a rwsem waiter can spin on the rwsem's owner
54 * and steal the lock, i.e. the lock is not anonymously owned.
55 * N.B. !owner is considered spinnable.
56 */
is_rwsem_owner_spinnable(struct task_struct * owner)57 static inline bool is_rwsem_owner_spinnable(struct task_struct *owner)
58 {
59 return !((unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED);
60 }
61
62 /*
63 * Return true if rwsem is owned by an anonymous writer or readers.
64 */
rwsem_has_anonymous_owner(struct task_struct * owner)65 static inline bool rwsem_has_anonymous_owner(struct task_struct *owner)
66 {
67 return (unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED;
68 }
69 #else
rwsem_set_owner(struct rw_semaphore * sem)70 static inline void rwsem_set_owner(struct rw_semaphore *sem)
71 {
72 }
73
rwsem_clear_owner(struct rw_semaphore * sem)74 static inline void rwsem_clear_owner(struct rw_semaphore *sem)
75 {
76 }
77
rwsem_set_reader_owned(struct rw_semaphore * sem)78 static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
79 {
80 }
81 #endif
82