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 consitent 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 { *(x) = (seqlock_t) __SEQLOCK_UNLOCKED(x); } while (0)
49
50 #define DEFINE_SEQLOCK(x) \
51 seqlock_t x = __SEQLOCK_UNLOCKED(x)
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 */
write_seqlock(seqlock_t * sl)57 static inline void write_seqlock(seqlock_t *sl)
58 {
59 spin_lock(&sl->lock);
60 ++sl->sequence;
61 smp_wmb();
62 }
63
write_sequnlock(seqlock_t * sl)64 static inline void write_sequnlock(seqlock_t *sl)
65 {
66 smp_wmb();
67 sl->sequence++;
68 spin_unlock(&sl->lock);
69 }
70
write_tryseqlock(seqlock_t * sl)71 static inline int write_tryseqlock(seqlock_t *sl)
72 {
73 int ret = spin_trylock(&sl->lock);
74
75 if (ret) {
76 ++sl->sequence;
77 smp_wmb();
78 }
79 return ret;
80 }
81
82 /* Start of read calculation -- fetch last complete writer token */
read_seqbegin(const seqlock_t * sl)83 static __always_inline unsigned read_seqbegin(const seqlock_t *sl)
84 {
85 unsigned ret = sl->sequence;
86 smp_rmb();
87 return ret;
88 }
89
90 /* Test if reader processed invalid data.
91 * If initial values is odd,
92 * then writer had already started when section was entered
93 * If sequence value changed
94 * then writer changed data while in section
95 *
96 * Using xor saves one conditional branch.
97 */
read_seqretry(const seqlock_t * sl,unsigned iv)98 static __always_inline int read_seqretry(const seqlock_t *sl, unsigned iv)
99 {
100 smp_rmb();
101 return (iv & 1) | (sl->sequence ^ iv);
102 }
103
104
105 /*
106 * Version using sequence counter only.
107 * This can be used when code has its own mutex protecting the
108 * updating starting before the write_seqcountbeqin() and ending
109 * after the write_seqcount_end().
110 */
111
112 typedef struct seqcount {
113 unsigned sequence;
114 } seqcount_t;
115
116 #define SEQCNT_ZERO { 0 }
117 #define seqcount_init(x) do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0)
118
119 /* Start of read using pointer to a sequence counter only. */
read_seqcount_begin(const seqcount_t * s)120 static inline unsigned read_seqcount_begin(const seqcount_t *s)
121 {
122 unsigned ret = s->sequence;
123 smp_rmb();
124 return ret;
125 }
126
127 /* Test if reader processed invalid data.
128 * Equivalent to: iv is odd or sequence number has changed.
129 * (iv & 1) || (*s != iv)
130 * Using xor saves one conditional branch.
131 */
read_seqcount_retry(const seqcount_t * s,unsigned iv)132 static inline int read_seqcount_retry(const seqcount_t *s, unsigned iv)
133 {
134 smp_rmb();
135 return (iv & 1) | (s->sequence ^ iv);
136 }
137
138
139 /*
140 * Sequence counter only version assumes that callers are using their
141 * own mutexing.
142 */
write_seqcount_begin(seqcount_t * s)143 static inline void write_seqcount_begin(seqcount_t *s)
144 {
145 s->sequence++;
146 smp_wmb();
147 }
148
write_seqcount_end(seqcount_t * s)149 static inline void write_seqcount_end(seqcount_t *s)
150 {
151 smp_wmb();
152 s->sequence++;
153 }
154
155 /*
156 * Possible sw/hw IRQ protected versions of the interfaces.
157 */
158 #define write_seqlock_irqsave(lock, flags) \
159 do { local_irq_save(flags); write_seqlock(lock); } while (0)
160 #define write_seqlock_irq(lock) \
161 do { local_irq_disable(); write_seqlock(lock); } while (0)
162 #define write_seqlock_bh(lock) \
163 do { local_bh_disable(); write_seqlock(lock); } while (0)
164
165 #define write_sequnlock_irqrestore(lock, flags) \
166 do { write_sequnlock(lock); local_irq_restore(flags); } while(0)
167 #define write_sequnlock_irq(lock) \
168 do { write_sequnlock(lock); local_irq_enable(); } while(0)
169 #define write_sequnlock_bh(lock) \
170 do { write_sequnlock(lock); local_bh_enable(); } while(0)
171
172 #define read_seqbegin_irqsave(lock, flags) \
173 ({ local_irq_save(flags); read_seqbegin(lock); })
174
175 #define read_seqretry_irqrestore(lock, iv, flags) \
176 ({ \
177 int ret = read_seqretry(lock, iv); \
178 local_irq_restore(flags); \
179 ret; \
180 })
181
182 #endif /* __LINUX_SEQLOCK_H */
183