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 */
198
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