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47a3a827 | 1 | // SPDX-License-Identifier: LGPL-2.1-or-later |
440d5faa DL |
2 | /* |
3 | * "Sequence" lock primitive | |
4 | * | |
5 | * Copyright (C) 2015 David Lamparter <equinox@diac24.net> | |
440d5faa DL |
6 | */ |
7 | ||
8 | #ifndef _SEQLOCK_H | |
9 | #define _SEQLOCK_H | |
10 | ||
11 | #include <stdbool.h> | |
12 | #include <stdint.h> | |
13 | #include <pthread.h> | |
14 | #include "frratomic.h" | |
15 | ||
17e38209 RW |
16 | #ifdef __cplusplus |
17 | extern "C" { | |
18 | #endif | |
19 | ||
440d5faa DL |
20 | /* |
21 | * this locking primitive is intended to use in a 1:N setup. | |
22 | * | |
23 | * - one "counter" seqlock issuing increasing numbers | |
24 | * - multiple seqlock users hold references on these numbers | |
25 | * | |
26 | * this is intended for implementing RCU reference-holding. There is one | |
27 | * global counter, with threads locking a seqlock whenever they take a | |
28 | * reference. A seqlock can also be idle/unlocked. | |
29 | * | |
30 | * The "counter" seqlock will always stay locked; the RCU cleanup thread | |
31 | * continuously counts it up, waiting for threads to release or progress to a | |
32 | * sequence number further ahead. If all threads are > N, references dropped | |
33 | * in N can be free'd. | |
34 | * | |
35 | * generally, the lock function is: | |
36 | * | |
37 | * Thread-A Thread-B | |
38 | * | |
39 | * seqlock_acquire(a) | |
40 | * | running seqlock_wait(b) -- a <= b | |
41 | * seqlock_release() | blocked | |
42 | * OR: seqlock_acquire(a') | -- a' > b | |
43 | * (resumes) | |
44 | */ | |
45 | ||
46 | /* use sequentially increasing "ticket numbers". lowest bit will always | |
6046b690 DL |
47 | * be 1 to have a 'cleared' indication (i.e., counts 1,5,9,13,etc. ) |
48 | * 2nd lowest bit is used to indicate we have waiters. | |
440d5faa DL |
49 | */ |
50 | typedef _Atomic uint32_t seqlock_ctr_t; | |
51 | typedef uint32_t seqlock_val_t; | |
6046b690 | 52 | #define seqlock_assert_valid(val) assert((val) & SEQLOCK_HELD) |
440d5faa | 53 | |
30ef834a DL |
54 | /* NB: SEQLOCK_WAITERS is only allowed if SEQLOCK_HELD is also set; can't |
55 | * have waiters on an unheld seqlock | |
56 | */ | |
6046b690 DL |
57 | #define SEQLOCK_HELD (1U << 0) |
58 | #define SEQLOCK_WAITERS (1U << 1) | |
59 | #define SEQLOCK_VAL(n) ((n) & ~SEQLOCK_WAITERS) | |
60 | #define SEQLOCK_STARTVAL 1U | |
61 | #define SEQLOCK_INCR 4U | |
440d5faa | 62 | |
30ef834a DL |
63 | /* TODO: originally, this was using "atomic_fetch_add", which is the reason |
64 | * bit 0 is used to indicate held state. With SEQLOCK_WAITERS added, there's | |
65 | * no fetch_add anymore (cmpxchg loop instead), so we don't need to use bit 0 | |
66 | * for this anymore & can just special-case the value 0 for it and skip it in | |
67 | * counting. | |
68 | */ | |
69 | ||
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70 | struct seqlock { |
71 | /* always used */ | |
72 | seqlock_ctr_t pos; | |
73 | /* used when futexes not available: (i.e. non-linux) */ | |
74 | pthread_mutex_t lock; | |
75 | pthread_cond_t wake; | |
76 | }; | |
77 | ||
78 | ||
79 | /* sqlo = 0 - init state: not held */ | |
80 | extern void seqlock_init(struct seqlock *sqlo); | |
81 | ||
82 | ||
30ef834a DL |
83 | /* basically: "while (sqlo <= val) wait();" |
84 | * returns when sqlo > val || !seqlock_held(sqlo) | |
85 | */ | |
440d5faa | 86 | extern void seqlock_wait(struct seqlock *sqlo, seqlock_val_t val); |
30ef834a DL |
87 | |
88 | /* same, but time-limited (limit is an absolute CLOCK_MONOTONIC value) */ | |
2a5e6235 DL |
89 | extern bool seqlock_timedwait(struct seqlock *sqlo, seqlock_val_t val, |
90 | const struct timespec *abs_monotime_limit); | |
30ef834a DL |
91 | |
92 | /* one-shot test, returns true if seqlock_wait would return immediately */ | |
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93 | extern bool seqlock_check(struct seqlock *sqlo, seqlock_val_t val); |
94 | ||
95 | static inline bool seqlock_held(struct seqlock *sqlo) | |
96 | { | |
97 | return !!atomic_load_explicit(&sqlo->pos, memory_order_relaxed); | |
98 | } | |
99 | ||
100 | /* sqlo - get seqlock position -- for the "counter" seqlock */ | |
101 | extern seqlock_val_t seqlock_cur(struct seqlock *sqlo); | |
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102 | |
103 | /* ++sqlo (but atomic & wakes waiters) - returns value that we bumped to. | |
104 | * | |
105 | * guarantees: | |
106 | * - each seqlock_bump call bumps the position by exactly one SEQLOCK_INCR. | |
107 | * There are no skipped/missed or multiple increments. | |
108 | * - each return value is only returned from one seqlock_bump() call | |
109 | */ | |
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110 | extern seqlock_val_t seqlock_bump(struct seqlock *sqlo); |
111 | ||
112 | ||
113 | /* sqlo = val - can be used on held seqlock. */ | |
114 | extern void seqlock_acquire_val(struct seqlock *sqlo, seqlock_val_t val); | |
30ef834a | 115 | |
440d5faa DL |
116 | /* sqlo = ref - standard pattern: acquire relative to other seqlock */ |
117 | static inline void seqlock_acquire(struct seqlock *sqlo, struct seqlock *ref) | |
118 | { | |
119 | seqlock_acquire_val(sqlo, seqlock_cur(ref)); | |
120 | } | |
121 | ||
122 | /* sqlo = 0 - set seqlock position to 0, marking as non-held */ | |
123 | extern void seqlock_release(struct seqlock *sqlo); | |
124 | /* release should normally be followed by a bump on the "counter", if | |
125 | * anything other than reading RCU items was done | |
126 | */ | |
127 | ||
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128 | #ifdef __cplusplus |
129 | } | |
130 | #endif | |
131 | ||
440d5faa | 132 | #endif /* _SEQLOCK_H */ |