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1 | #ifndef __LINUX_SEQLOCK_H | |
2 | #define __LINUX_SEQLOCK_H | |
3 | /* | |
4 | * Reader/writer consistent mechanism without starving writers. This type of | |
5 | * lock for data where the reader wants a consistent set of information | |
6 | * and is willing to retry if the information changes. Readers never | |
7 | * block but they may have to retry if a writer is in | |
8 | * progress. Writers do not wait for readers. | |
9 | * | |
10 | * This is not as cache friendly as brlock. Also, this will not work | |
11 | * for data that contains pointers, because any writer could | |
12 | * invalidate a pointer that a reader was following. | |
13 | * | |
14 | * Expected reader usage: | |
15 | * do { | |
16 | * seq = read_seqbegin(&foo); | |
17 | * ... | |
18 | * } while (read_seqretry(&foo, seq)); | |
19 | * | |
20 | * | |
21 | * On non-SMP the spin locks disappear but the writer still needs | |
22 | * to increment the sequence variables because an interrupt routine could | |
23 | * change the state of the data. | |
24 | * | |
25 | * Based on x86_64 vsyscall gettimeofday | |
26 | * by Keith Owens and Andrea Arcangeli | |
27 | */ | |
28 | ||
29 | #include <linux/spinlock.h> | |
30 | #include <linux/preempt.h> | |
31 | ||
32 | typedef struct { | |
33 | unsigned sequence; | |
34 | spinlock_t lock; | |
35 | } seqlock_t; | |
36 | ||
37 | /* | |
38 | * These macros triggered gcc-3.x compile-time problems. We think these are | |
39 | * OK now. Be cautious. | |
40 | */ | |
41 | #define __SEQLOCK_UNLOCKED(lockname) \ | |
42 | { 0, __SPIN_LOCK_UNLOCKED(lockname) } | |
43 | ||
44 | #define SEQLOCK_UNLOCKED \ | |
45 | __SEQLOCK_UNLOCKED(old_style_seqlock_init) | |
46 | ||
47 | #define seqlock_init(x) \ | |
48 | do { \ | |
49 | (x)->sequence = 0; \ | |
50 | spin_lock_init(&(x)->lock); \ | |
51 | } while (0) | |
52 | ||
53 | #define DEFINE_SEQLOCK(x) \ | |
54 | seqlock_t x = __SEQLOCK_UNLOCKED(x) | |
55 | ||
56 | /* Lock out other writers and update the count. | |
57 | * Acts like a normal spin_lock/unlock. | |
58 | * Don't need preempt_disable() because that is in the spin_lock already. | |
59 | */ | |
60 | static inline void write_seqlock(seqlock_t *sl) | |
61 | { | |
62 | spin_lock(&sl->lock); | |
63 | ++sl->sequence; | |
64 | smp_wmb(); | |
65 | } | |
66 | ||
67 | static inline void write_sequnlock(seqlock_t *sl) | |
68 | { | |
69 | smp_wmb(); | |
70 | sl->sequence++; | |
71 | spin_unlock(&sl->lock); | |
72 | } | |
73 | ||
74 | static inline int write_tryseqlock(seqlock_t *sl) | |
75 | { | |
76 | int ret = spin_trylock(&sl->lock); | |
77 | ||
78 | if (ret) { | |
79 | ++sl->sequence; | |
80 | smp_wmb(); | |
81 | } | |
82 | return ret; | |
83 | } | |
84 | ||
85 | /* Start of read calculation -- fetch last complete writer token */ | |
86 | static __always_inline unsigned read_seqbegin(const seqlock_t *sl) | |
87 | { | |
88 | unsigned ret; | |
89 | ||
90 | repeat: | |
91 | ret = sl->sequence; | |
92 | smp_rmb(); | |
93 | if (unlikely(ret & 1)) { | |
94 | cpu_relax(); | |
95 | goto repeat; | |
96 | } | |
97 | ||
98 | return ret; | |
99 | } | |
100 | ||
101 | /* | |
102 | * Test if reader processed invalid data. | |
103 | * | |
104 | * If sequence value changed then writer changed data while in section. | |
105 | */ | |
106 | static __always_inline int read_seqretry(const seqlock_t *sl, unsigned start) | |
107 | { | |
108 | smp_rmb(); | |
109 | ||
110 | return (sl->sequence != start); | |
111 | } | |
112 | ||
113 | ||
114 | /* | |
115 | * Version using sequence counter only. | |
116 | * This can be used when code has its own mutex protecting the | |
117 | * updating starting before the write_seqcountbeqin() and ending | |
118 | * after the write_seqcount_end(). | |
119 | */ | |
120 | ||
121 | typedef struct seqcount { | |
122 | unsigned sequence; | |
123 | } seqcount_t; | |
124 | ||
125 | #define SEQCNT_ZERO { 0 } | |
126 | #define seqcount_init(x) do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0) | |
127 | ||
128 | /* Start of read using pointer to a sequence counter only. */ | |
129 | static inline unsigned read_seqcount_begin(const seqcount_t *s) | |
130 | { | |
131 | unsigned ret; | |
132 | ||
133 | repeat: | |
134 | ret = s->sequence; | |
135 | smp_rmb(); | |
136 | if (unlikely(ret & 1)) { | |
137 | cpu_relax(); | |
138 | goto repeat; | |
139 | } | |
140 | return ret; | |
141 | } | |
142 | ||
143 | /* | |
144 | * Test if reader processed invalid data because sequence number has changed. | |
145 | */ | |
146 | static inline int read_seqcount_retry(const seqcount_t *s, unsigned start) | |
147 | { | |
148 | smp_rmb(); | |
149 | ||
150 | return s->sequence != start; | |
151 | } | |
152 | ||
153 | ||
154 | /* | |
155 | * Sequence counter only version assumes that callers are using their | |
156 | * own mutexing. | |
157 | */ | |
158 | static inline void write_seqcount_begin(seqcount_t *s) | |
159 | { | |
160 | s->sequence++; | |
161 | smp_wmb(); | |
162 | } | |
163 | ||
164 | static inline void write_seqcount_end(seqcount_t *s) | |
165 | { | |
166 | smp_wmb(); | |
167 | s->sequence++; | |
168 | } | |
169 | ||
170 | /* | |
171 | * Possible sw/hw IRQ protected versions of the interfaces. | |
172 | */ | |
173 | #define write_seqlock_irqsave(lock, flags) \ | |
174 | do { local_irq_save(flags); write_seqlock(lock); } while (0) | |
175 | #define write_seqlock_irq(lock) \ | |
176 | do { local_irq_disable(); write_seqlock(lock); } while (0) | |
177 | #define write_seqlock_bh(lock) \ | |
178 | do { local_bh_disable(); write_seqlock(lock); } while (0) | |
179 | ||
180 | #define write_sequnlock_irqrestore(lock, flags) \ | |
181 | do { write_sequnlock(lock); local_irq_restore(flags); } while(0) | |
182 | #define write_sequnlock_irq(lock) \ | |
183 | do { write_sequnlock(lock); local_irq_enable(); } while(0) | |
184 | #define write_sequnlock_bh(lock) \ | |
185 | do { write_sequnlock(lock); local_bh_enable(); } while(0) | |
186 | ||
187 | #define read_seqbegin_irqsave(lock, flags) \ | |
188 | ({ local_irq_save(flags); read_seqbegin(lock); }) | |
189 | ||
190 | #define read_seqretry_irqrestore(lock, iv, flags) \ | |
191 | ({ \ | |
192 | int ret = read_seqretry(lock, iv); \ | |
193 | local_irq_restore(flags); \ | |
194 | ret; \ | |
195 | }) | |
196 | ||
197 | #endif /* __LINUX_SEQLOCK_H */ |