06d69648fc86c3a731c528cde7fba3b72a2688be
[linux-2.6.git] / include / linux / seqlock.h
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 = ACCESS_ONCE(sl->sequence);
92         if (unlikely(ret & 1)) {
93                 cpu_relax();
94                 goto repeat;
95         }
96         smp_rmb();
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 unlikely(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 /**
129  * __read_seqcount_begin - begin a seq-read critical section (without barrier)
130  * @s: pointer to seqcount_t
131  * Returns: count to be passed to read_seqcount_retry
132  *
133  * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
134  * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
135  * provided before actually loading any of the variables that are to be
136  * protected in this critical section.
137  *
138  * Use carefully, only in critical code, and comment how the barrier is
139  * provided.
140  */
141 static inline unsigned __read_seqcount_begin(const seqcount_t *s)
142 {
143         unsigned ret;
144
145 repeat:
146         ret = s->sequence;
147         if (unlikely(ret & 1)) {
148                 cpu_relax();
149                 goto repeat;
150         }
151         return ret;
152 }
153
154 /**
155  * read_seqcount_begin - begin a seq-read critical section
156  * @s: pointer to seqcount_t
157  * Returns: count to be passed to read_seqcount_retry
158  *
159  * read_seqcount_begin opens a read critical section of the given seqcount.
160  * Validity of the critical section is tested by checking read_seqcount_retry
161  * function.
162  */
163 static inline unsigned read_seqcount_begin(const seqcount_t *s)
164 {
165         unsigned ret = __read_seqcount_begin(s);
166         smp_rmb();
167         return ret;
168 }
169
170 /**
171  * __read_seqcount_retry - end a seq-read critical section (without barrier)
172  * @s: pointer to seqcount_t
173  * @start: count, from read_seqcount_begin
174  * Returns: 1 if retry is required, else 0
175  *
176  * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
177  * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
178  * provided before actually loading any of the variables that are to be
179  * protected in this critical section.
180  *
181  * Use carefully, only in critical code, and comment how the barrier is
182  * provided.
183  */
184 static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
185 {
186         return unlikely(s->sequence != start);
187 }
188
189 /**
190  * read_seqcount_retry - end a seq-read critical section
191  * @s: pointer to seqcount_t
192  * @start: count, from read_seqcount_begin
193  * Returns: 1 if retry is required, else 0
194  *
195  * read_seqcount_retry closes a read critical section of the given seqcount.
196  * If the critical section was invalid, it must be ignored (and typically
197  * retried).
198  */
199 static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
200 {
201         smp_rmb();
202
203         return __read_seqcount_retry(s, start);
204 }
205
206
207 /*
208  * Sequence counter only version assumes that callers are using their
209  * own mutexing.
210  */
211 static inline void write_seqcount_begin(seqcount_t *s)
212 {
213         s->sequence++;
214         smp_wmb();
215 }
216
217 static inline void write_seqcount_end(seqcount_t *s)
218 {
219         smp_wmb();
220         s->sequence++;
221 }
222
223 /**
224  * write_seqcount_barrier - invalidate in-progress read-side seq operations
225  * @s: pointer to seqcount_t
226  *
227  * After write_seqcount_barrier, no read-side seq operations will complete
228  * successfully and see data older than this.
229  */
230 static inline void write_seqcount_barrier(seqcount_t *s)
231 {
232         smp_wmb();
233         s->sequence+=2;
234 }
235
236 /*
237  * Possible sw/hw IRQ protected versions of the interfaces.
238  */
239 #define write_seqlock_irqsave(lock, flags)                              \
240         do { local_irq_save(flags); write_seqlock(lock); } while (0)
241 #define write_seqlock_irq(lock)                                         \
242         do { local_irq_disable();   write_seqlock(lock); } while (0)
243 #define write_seqlock_bh(lock)                                          \
244         do { local_bh_disable();    write_seqlock(lock); } while (0)
245
246 #define write_sequnlock_irqrestore(lock, flags)                         \
247         do { write_sequnlock(lock); local_irq_restore(flags); } while(0)
248 #define write_sequnlock_irq(lock)                                       \
249         do { write_sequnlock(lock); local_irq_enable(); } while(0)
250 #define write_sequnlock_bh(lock)                                        \
251         do { write_sequnlock(lock); local_bh_enable(); } while(0)
252
253 #define read_seqbegin_irqsave(lock, flags)                              \
254         ({ local_irq_save(flags);   read_seqbegin(lock); })
255
256 #define read_seqretry_irqrestore(lock, iv, flags)                       \
257         ({                                                              \
258                 int ret = read_seqretry(lock, iv);                      \
259                 local_irq_restore(flags);                               \
260                 ret;                                                    \
261         })
262
263 #endif /* __LINUX_SEQLOCK_H */