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_DEBUG_RWSEMS
24 # define DEBUG_RWSEMS_WARN_ON(c) DEBUG_LOCKS_WARN_ON(c)
25 #else
26 # define DEBUG_RWSEMS_WARN_ON(c)
27 #endif
28
29 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
30 /*
31 * All writes to owner are protected by WRITE_ONCE() to make sure that
32 * store tearing can't happen as optimistic spinners may read and use
33 * the owner value concurrently without lock. Read from owner, however,
34 * may not need READ_ONCE() as long as the pointer value is only used
35 * for comparison and isn't being dereferenced.
36 */
rwsem_set_owner(struct rw_semaphore * sem)37 static inline void rwsem_set_owner(struct rw_semaphore *sem)
38 {
39 WRITE_ONCE(sem->owner, current);
40 }
41
rwsem_clear_owner(struct rw_semaphore * sem)42 static inline void rwsem_clear_owner(struct rw_semaphore *sem)
43 {
44 WRITE_ONCE(sem->owner, NULL);
45 }
46
rwsem_set_reader_owned(struct rw_semaphore * sem)47 static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
48 {
49 /*
50 * We check the owner value first to make sure that we will only
51 * do a write to the rwsem cacheline when it is really necessary
52 * to minimize cacheline contention.
53 */
54 if (READ_ONCE(sem->owner) != RWSEM_READER_OWNED)
55 WRITE_ONCE(sem->owner, RWSEM_READER_OWNED);
56 }
57
58 /*
59 * Return true if the a rwsem waiter can spin on the rwsem's owner
60 * and steal the lock, i.e. the lock is not anonymously owned.
61 * N.B. !owner is considered spinnable.
62 */
is_rwsem_owner_spinnable(struct task_struct * owner)63 static inline bool is_rwsem_owner_spinnable(struct task_struct *owner)
64 {
65 return !((unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED);
66 }
67
68 /*
69 * Return true if rwsem is owned by an anonymous writer or readers.
70 */
rwsem_has_anonymous_owner(struct task_struct * owner)71 static inline bool rwsem_has_anonymous_owner(struct task_struct *owner)
72 {
73 return (unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED;
74 }
75 #else
rwsem_set_owner(struct rw_semaphore * sem)76 static inline void rwsem_set_owner(struct rw_semaphore *sem)
77 {
78 }
79
rwsem_clear_owner(struct rw_semaphore * sem)80 static inline void rwsem_clear_owner(struct rw_semaphore *sem)
81 {
82 }
83
rwsem_set_reader_owned(struct rw_semaphore * sem)84 static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
85 {
86 }
87 #endif
88
