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1da177e4 LT |
1 | #ifndef __LINUX_SEQLOCK_H |
2 | #define __LINUX_SEQLOCK_H | |
3 | /* | |
4 | * Reader/writer consistent mechanism without starving writers. This type of | |
d08df601 | 5 | * lock for data where the reader wants a consistent set of information |
1da177e4 LT |
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 | ||
1da177e4 LT |
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 | */ | |
e4d91918 IM |
41 | #define __SEQLOCK_UNLOCKED(lockname) \ |
42 | { 0, __SPIN_LOCK_UNLOCKED(lockname) } | |
1da177e4 | 43 | |
99a3eb38 IM |
44 | #define seqlock_init(x) \ |
45 | do { \ | |
46 | (x)->sequence = 0; \ | |
47 | spin_lock_init(&(x)->lock); \ | |
48 | } while (0) | |
e4d91918 IM |
49 | |
50 | #define DEFINE_SEQLOCK(x) \ | |
51 | seqlock_t x = __SEQLOCK_UNLOCKED(x) | |
1da177e4 LT |
52 | |
53 | /* Lock out other writers and update the count. | |
54 | * Acts like a normal spin_lock/unlock. | |
55 | * Don't need preempt_disable() because that is in the spin_lock already. | |
56 | */ | |
57 | static inline void write_seqlock(seqlock_t *sl) | |
58 | { | |
59 | spin_lock(&sl->lock); | |
60 | ++sl->sequence; | |
20f09390 DW |
61 | smp_wmb(); |
62 | } | |
1da177e4 | 63 | |
20f09390 | 64 | static inline void write_sequnlock(seqlock_t *sl) |
1da177e4 LT |
65 | { |
66 | smp_wmb(); | |
67 | sl->sequence++; | |
68 | spin_unlock(&sl->lock); | |
69 | } | |
70 | ||
71 | static inline int write_tryseqlock(seqlock_t *sl) | |
72 | { | |
73 | int ret = spin_trylock(&sl->lock); | |
74 | ||
75 | if (ret) { | |
76 | ++sl->sequence; | |
20f09390 | 77 | smp_wmb(); |
1da177e4 LT |
78 | } |
79 | return ret; | |
80 | } | |
81 | ||
82 | /* Start of read calculation -- fetch last complete writer token */ | |
cde227af | 83 | static __always_inline unsigned read_seqbegin(const seqlock_t *sl) |
1da177e4 | 84 | { |
88a411c0 IM |
85 | unsigned ret; |
86 | ||
87 | repeat: | |
5db1256a | 88 | ret = ACCESS_ONCE(sl->sequence); |
88a411c0 IM |
89 | if (unlikely(ret & 1)) { |
90 | cpu_relax(); | |
91 | goto repeat; | |
92 | } | |
5db1256a | 93 | smp_rmb(); |
88a411c0 | 94 | |
1da177e4 LT |
95 | return ret; |
96 | } | |
97 | ||
88a411c0 IM |
98 | /* |
99 | * Test if reader processed invalid data. | |
100 | * | |
101 | * If sequence value changed then writer changed data while in section. | |
1da177e4 | 102 | */ |
88a411c0 | 103 | static __always_inline int read_seqretry(const seqlock_t *sl, unsigned start) |
1da177e4 LT |
104 | { |
105 | smp_rmb(); | |
88a411c0 | 106 | |
3c22cd57 | 107 | return unlikely(sl->sequence != start); |
1da177e4 LT |
108 | } |
109 | ||
110 | ||
111 | /* | |
112 | * Version using sequence counter only. | |
113 | * This can be used when code has its own mutex protecting the | |
114 | * updating starting before the write_seqcountbeqin() and ending | |
115 | * after the write_seqcount_end(). | |
116 | */ | |
117 | ||
118 | typedef struct seqcount { | |
119 | unsigned sequence; | |
120 | } seqcount_t; | |
121 | ||
122 | #define SEQCNT_ZERO { 0 } | |
123 | #define seqcount_init(x) do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0) | |
124 | ||
3c22cd57 NP |
125 | /** |
126 | * __read_seqcount_begin - begin a seq-read critical section (without barrier) | |
127 | * @s: pointer to seqcount_t | |
128 | * Returns: count to be passed to read_seqcount_retry | |
129 | * | |
130 | * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb() | |
131 | * barrier. Callers should ensure that smp_rmb() or equivalent ordering is | |
132 | * provided before actually loading any of the variables that are to be | |
133 | * protected in this critical section. | |
134 | * | |
135 | * Use carefully, only in critical code, and comment how the barrier is | |
136 | * provided. | |
137 | */ | |
138 | static inline unsigned __read_seqcount_begin(const seqcount_t *s) | |
1da177e4 | 139 | { |
88a411c0 IM |
140 | unsigned ret; |
141 | ||
142 | repeat: | |
143 | ret = s->sequence; | |
88a411c0 IM |
144 | if (unlikely(ret & 1)) { |
145 | cpu_relax(); | |
146 | goto repeat; | |
147 | } | |
1da177e4 LT |
148 | return ret; |
149 | } | |
150 | ||
3c22cd57 NP |
151 | /** |
152 | * read_seqcount_begin - begin a seq-read critical section | |
153 | * @s: pointer to seqcount_t | |
154 | * Returns: count to be passed to read_seqcount_retry | |
155 | * | |
156 | * read_seqcount_begin opens a read critical section of the given seqcount. | |
157 | * Validity of the critical section is tested by checking read_seqcount_retry | |
158 | * function. | |
159 | */ | |
160 | static inline unsigned read_seqcount_begin(const seqcount_t *s) | |
161 | { | |
162 | unsigned ret = __read_seqcount_begin(s); | |
163 | smp_rmb(); | |
164 | return ret; | |
165 | } | |
166 | ||
167 | /** | |
168 | * __read_seqcount_retry - end a seq-read critical section (without barrier) | |
169 | * @s: pointer to seqcount_t | |
170 | * @start: count, from read_seqcount_begin | |
171 | * Returns: 1 if retry is required, else 0 | |
172 | * | |
173 | * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb() | |
174 | * barrier. Callers should ensure that smp_rmb() or equivalent ordering is | |
175 | * provided before actually loading any of the variables that are to be | |
176 | * protected in this critical section. | |
177 | * | |
178 | * Use carefully, only in critical code, and comment how the barrier is | |
179 | * provided. | |
180 | */ | |
181 | static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start) | |
182 | { | |
183 | return unlikely(s->sequence != start); | |
184 | } | |
185 | ||
186 | /** | |
187 | * read_seqcount_retry - end a seq-read critical section | |
188 | * @s: pointer to seqcount_t | |
189 | * @start: count, from read_seqcount_begin | |
190 | * Returns: 1 if retry is required, else 0 | |
191 | * | |
192 | * read_seqcount_retry closes a read critical section of the given seqcount. | |
193 | * If the critical section was invalid, it must be ignored (and typically | |
194 | * retried). | |
1da177e4 | 195 | */ |
88a411c0 | 196 | static inline int read_seqcount_retry(const seqcount_t *s, unsigned start) |
1da177e4 LT |
197 | { |
198 | smp_rmb(); | |
88a411c0 | 199 | |
3c22cd57 | 200 | return __read_seqcount_retry(s, start); |
1da177e4 LT |
201 | } |
202 | ||
203 | ||
204 | /* | |
205 | * Sequence counter only version assumes that callers are using their | |
206 | * own mutexing. | |
207 | */ | |
208 | static inline void write_seqcount_begin(seqcount_t *s) | |
209 | { | |
210 | s->sequence++; | |
211 | smp_wmb(); | |
212 | } | |
213 | ||
214 | static inline void write_seqcount_end(seqcount_t *s) | |
215 | { | |
216 | smp_wmb(); | |
217 | s->sequence++; | |
218 | } | |
219 | ||
3c22cd57 NP |
220 | /** |
221 | * write_seqcount_barrier - invalidate in-progress read-side seq operations | |
222 | * @s: pointer to seqcount_t | |
223 | * | |
224 | * After write_seqcount_barrier, no read-side seq operations will complete | |
225 | * successfully and see data older than this. | |
226 | */ | |
227 | static inline void write_seqcount_barrier(seqcount_t *s) | |
228 | { | |
229 | smp_wmb(); | |
230 | s->sequence+=2; | |
231 | } | |
232 | ||
1da177e4 LT |
233 | /* |
234 | * Possible sw/hw IRQ protected versions of the interfaces. | |
235 | */ | |
236 | #define write_seqlock_irqsave(lock, flags) \ | |
237 | do { local_irq_save(flags); write_seqlock(lock); } while (0) | |
238 | #define write_seqlock_irq(lock) \ | |
239 | do { local_irq_disable(); write_seqlock(lock); } while (0) | |
240 | #define write_seqlock_bh(lock) \ | |
241 | do { local_bh_disable(); write_seqlock(lock); } while (0) | |
242 | ||
243 | #define write_sequnlock_irqrestore(lock, flags) \ | |
244 | do { write_sequnlock(lock); local_irq_restore(flags); } while(0) | |
245 | #define write_sequnlock_irq(lock) \ | |
246 | do { write_sequnlock(lock); local_irq_enable(); } while(0) | |
247 | #define write_sequnlock_bh(lock) \ | |
248 | do { write_sequnlock(lock); local_bh_enable(); } while(0) | |
249 | ||
250 | #define read_seqbegin_irqsave(lock, flags) \ | |
251 | ({ local_irq_save(flags); read_seqbegin(lock); }) | |
252 | ||
253 | #define read_seqretry_irqrestore(lock, iv, flags) \ | |
254 | ({ \ | |
255 | int ret = read_seqretry(lock, iv); \ | |
256 | local_irq_restore(flags); \ | |
257 | ret; \ | |
258 | }) | |
259 | ||
260 | #endif /* __LINUX_SEQLOCK_H */ |