Commit | Line | Data |
---|---|---|
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> | |
56a21052 | 31 | #include <asm/processor.h> |
1da177e4 | 32 | |
1da177e4 LT |
33 | /* |
34 | * Version using sequence counter only. | |
35 | * This can be used when code has its own mutex protecting the | |
36 | * updating starting before the write_seqcountbeqin() and ending | |
37 | * after the write_seqcount_end(). | |
38 | */ | |
1da177e4 LT |
39 | typedef struct seqcount { |
40 | unsigned sequence; | |
41 | } seqcount_t; | |
42 | ||
43 | #define SEQCNT_ZERO { 0 } | |
44 | #define seqcount_init(x) do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0) | |
45 | ||
3c22cd57 NP |
46 | /** |
47 | * __read_seqcount_begin - begin a seq-read critical section (without barrier) | |
48 | * @s: pointer to seqcount_t | |
49 | * Returns: count to be passed to read_seqcount_retry | |
50 | * | |
51 | * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb() | |
52 | * barrier. Callers should ensure that smp_rmb() or equivalent ordering is | |
53 | * provided before actually loading any of the variables that are to be | |
54 | * protected in this critical section. | |
55 | * | |
56 | * Use carefully, only in critical code, and comment how the barrier is | |
57 | * provided. | |
58 | */ | |
59 | static inline unsigned __read_seqcount_begin(const seqcount_t *s) | |
1da177e4 | 60 | { |
88a411c0 IM |
61 | unsigned ret; |
62 | ||
63 | repeat: | |
2f624278 | 64 | ret = ACCESS_ONCE(s->sequence); |
88a411c0 IM |
65 | if (unlikely(ret & 1)) { |
66 | cpu_relax(); | |
67 | goto repeat; | |
68 | } | |
1da177e4 LT |
69 | return ret; |
70 | } | |
71 | ||
3c22cd57 NP |
72 | /** |
73 | * read_seqcount_begin - begin a seq-read critical section | |
74 | * @s: pointer to seqcount_t | |
75 | * Returns: count to be passed to read_seqcount_retry | |
76 | * | |
77 | * read_seqcount_begin opens a read critical section of the given seqcount. | |
78 | * Validity of the critical section is tested by checking read_seqcount_retry | |
79 | * function. | |
80 | */ | |
81 | static inline unsigned read_seqcount_begin(const seqcount_t *s) | |
82 | { | |
83 | unsigned ret = __read_seqcount_begin(s); | |
84 | smp_rmb(); | |
85 | return ret; | |
86 | } | |
87 | ||
4f988f15 LT |
88 | /** |
89 | * raw_seqcount_begin - begin a seq-read critical section | |
90 | * @s: pointer to seqcount_t | |
91 | * Returns: count to be passed to read_seqcount_retry | |
92 | * | |
93 | * raw_seqcount_begin opens a read critical section of the given seqcount. | |
94 | * Validity of the critical section is tested by checking read_seqcount_retry | |
95 | * function. | |
96 | * | |
97 | * Unlike read_seqcount_begin(), this function will not wait for the count | |
98 | * to stabilize. If a writer is active when we begin, we will fail the | |
99 | * read_seqcount_retry() instead of stabilizing at the beginning of the | |
100 | * critical section. | |
101 | */ | |
102 | static inline unsigned raw_seqcount_begin(const seqcount_t *s) | |
103 | { | |
104 | unsigned ret = ACCESS_ONCE(s->sequence); | |
105 | smp_rmb(); | |
106 | return ret & ~1; | |
107 | } | |
108 | ||
3c22cd57 NP |
109 | /** |
110 | * __read_seqcount_retry - end a seq-read critical section (without barrier) | |
111 | * @s: pointer to seqcount_t | |
112 | * @start: count, from read_seqcount_begin | |
113 | * Returns: 1 if retry is required, else 0 | |
114 | * | |
115 | * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb() | |
116 | * barrier. Callers should ensure that smp_rmb() or equivalent ordering is | |
117 | * provided before actually loading any of the variables that are to be | |
118 | * protected in this critical section. | |
119 | * | |
120 | * Use carefully, only in critical code, and comment how the barrier is | |
121 | * provided. | |
122 | */ | |
123 | static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start) | |
124 | { | |
125 | return unlikely(s->sequence != start); | |
126 | } | |
127 | ||
128 | /** | |
129 | * read_seqcount_retry - end a seq-read critical section | |
130 | * @s: pointer to seqcount_t | |
131 | * @start: count, from read_seqcount_begin | |
132 | * Returns: 1 if retry is required, else 0 | |
133 | * | |
134 | * read_seqcount_retry closes a read critical section of the given seqcount. | |
135 | * If the critical section was invalid, it must be ignored (and typically | |
136 | * retried). | |
1da177e4 | 137 | */ |
88a411c0 | 138 | static inline int read_seqcount_retry(const seqcount_t *s, unsigned start) |
1da177e4 LT |
139 | { |
140 | smp_rmb(); | |
3c22cd57 | 141 | return __read_seqcount_retry(s, start); |
1da177e4 LT |
142 | } |
143 | ||
144 | ||
145 | /* | |
146 | * Sequence counter only version assumes that callers are using their | |
147 | * own mutexing. | |
148 | */ | |
149 | static inline void write_seqcount_begin(seqcount_t *s) | |
150 | { | |
151 | s->sequence++; | |
152 | smp_wmb(); | |
153 | } | |
154 | ||
155 | static inline void write_seqcount_end(seqcount_t *s) | |
156 | { | |
157 | smp_wmb(); | |
158 | s->sequence++; | |
159 | } | |
160 | ||
3c22cd57 NP |
161 | /** |
162 | * write_seqcount_barrier - invalidate in-progress read-side seq operations | |
163 | * @s: pointer to seqcount_t | |
164 | * | |
165 | * After write_seqcount_barrier, no read-side seq operations will complete | |
166 | * successfully and see data older than this. | |
167 | */ | |
168 | static inline void write_seqcount_barrier(seqcount_t *s) | |
169 | { | |
170 | smp_wmb(); | |
171 | s->sequence+=2; | |
172 | } | |
173 | ||
6617feca TG |
174 | typedef struct { |
175 | struct seqcount seqcount; | |
176 | spinlock_t lock; | |
177 | } seqlock_t; | |
178 | ||
179 | /* | |
180 | * These macros triggered gcc-3.x compile-time problems. We think these are | |
181 | * OK now. Be cautious. | |
182 | */ | |
183 | #define __SEQLOCK_UNLOCKED(lockname) \ | |
184 | { \ | |
185 | .seqcount = SEQCNT_ZERO, \ | |
186 | .lock = __SPIN_LOCK_UNLOCKED(lockname) \ | |
187 | } | |
188 | ||
189 | #define seqlock_init(x) \ | |
190 | do { \ | |
191 | seqcount_init(&(x)->seqcount); \ | |
192 | spin_lock_init(&(x)->lock); \ | |
193 | } while (0) | |
194 | ||
195 | #define DEFINE_SEQLOCK(x) \ | |
196 | seqlock_t x = __SEQLOCK_UNLOCKED(x) | |
197 | ||
198 | /* | |
199 | * Read side functions for starting and finalizing a read side section. | |
200 | */ | |
201 | static inline unsigned read_seqbegin(const seqlock_t *sl) | |
202 | { | |
203 | return read_seqcount_begin(&sl->seqcount); | |
204 | } | |
205 | ||
206 | static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start) | |
207 | { | |
208 | return read_seqcount_retry(&sl->seqcount, start); | |
209 | } | |
210 | ||
1da177e4 | 211 | /* |
6617feca TG |
212 | * Lock out other writers and update the count. |
213 | * Acts like a normal spin_lock/unlock. | |
214 | * Don't need preempt_disable() because that is in the spin_lock already. | |
1da177e4 | 215 | */ |
6617feca TG |
216 | static inline void write_seqlock(seqlock_t *sl) |
217 | { | |
218 | spin_lock(&sl->lock); | |
219 | write_seqcount_begin(&sl->seqcount); | |
220 | } | |
221 | ||
222 | static inline void write_sequnlock(seqlock_t *sl) | |
223 | { | |
224 | write_seqcount_end(&sl->seqcount); | |
225 | spin_unlock(&sl->lock); | |
226 | } | |
227 | ||
228 | static inline void write_seqlock_bh(seqlock_t *sl) | |
229 | { | |
230 | spin_lock_bh(&sl->lock); | |
231 | write_seqcount_begin(&sl->seqcount); | |
232 | } | |
233 | ||
234 | static inline void write_sequnlock_bh(seqlock_t *sl) | |
235 | { | |
236 | write_seqcount_end(&sl->seqcount); | |
237 | spin_unlock_bh(&sl->lock); | |
238 | } | |
239 | ||
240 | static inline void write_seqlock_irq(seqlock_t *sl) | |
241 | { | |
242 | spin_lock_irq(&sl->lock); | |
243 | write_seqcount_begin(&sl->seqcount); | |
244 | } | |
245 | ||
246 | static inline void write_sequnlock_irq(seqlock_t *sl) | |
247 | { | |
248 | write_seqcount_end(&sl->seqcount); | |
249 | spin_unlock_irq(&sl->lock); | |
250 | } | |
251 | ||
252 | static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl) | |
253 | { | |
254 | unsigned long flags; | |
255 | ||
256 | spin_lock_irqsave(&sl->lock, flags); | |
257 | write_seqcount_begin(&sl->seqcount); | |
258 | return flags; | |
259 | } | |
260 | ||
1da177e4 | 261 | #define write_seqlock_irqsave(lock, flags) \ |
6617feca | 262 | do { flags = __write_seqlock_irqsave(lock); } while (0) |
1da177e4 | 263 | |
6617feca TG |
264 | static inline void |
265 | write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags) | |
266 | { | |
267 | write_seqcount_end(&sl->seqcount); | |
268 | spin_unlock_irqrestore(&sl->lock, flags); | |
269 | } | |
1da177e4 | 270 | |
1da177e4 | 271 | #endif /* __LINUX_SEQLOCK_H */ |