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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 | |
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/config.h> | |
30 | #include <linux/spinlock.h> | |
31 | #include <linux/preempt.h> | |
32 | ||
33 | typedef struct { | |
34 | unsigned sequence; | |
35 | spinlock_t lock; | |
36 | } seqlock_t; | |
37 | ||
38 | /* | |
39 | * These macros triggered gcc-3.x compile-time problems. We think these are | |
40 | * OK now. Be cautious. | |
41 | */ | |
42 | #define SEQLOCK_UNLOCKED { 0, SPIN_LOCK_UNLOCKED } | |
43 | #define seqlock_init(x) do { *(x) = (seqlock_t) SEQLOCK_UNLOCKED; } while (0) | |
44 | ||
45 | ||
46 | /* Lock out other writers and update the count. | |
47 | * Acts like a normal spin_lock/unlock. | |
48 | * Don't need preempt_disable() because that is in the spin_lock already. | |
49 | */ | |
50 | static inline void write_seqlock(seqlock_t *sl) | |
51 | { | |
52 | spin_lock(&sl->lock); | |
53 | ++sl->sequence; | |
54 | smp_wmb(); | |
55 | } | |
56 | ||
57 | static inline void write_sequnlock(seqlock_t *sl) | |
58 | { | |
59 | smp_wmb(); | |
60 | sl->sequence++; | |
61 | spin_unlock(&sl->lock); | |
62 | } | |
63 | ||
64 | static inline int write_tryseqlock(seqlock_t *sl) | |
65 | { | |
66 | int ret = spin_trylock(&sl->lock); | |
67 | ||
68 | if (ret) { | |
69 | ++sl->sequence; | |
70 | smp_wmb(); | |
71 | } | |
72 | return ret; | |
73 | } | |
74 | ||
75 | /* Start of read calculation -- fetch last complete writer token */ | |
76 | static inline unsigned read_seqbegin(const seqlock_t *sl) | |
77 | { | |
78 | unsigned ret = sl->sequence; | |
79 | smp_rmb(); | |
80 | return ret; | |
81 | } | |
82 | ||
83 | /* Test if reader processed invalid data. | |
84 | * If initial values is odd, | |
85 | * then writer had already started when section was entered | |
86 | * If sequence value changed | |
87 | * then writer changed data while in section | |
88 | * | |
89 | * Using xor saves one conditional branch. | |
90 | */ | |
91 | static inline int read_seqretry(const seqlock_t *sl, unsigned iv) | |
92 | { | |
93 | smp_rmb(); | |
94 | return (iv & 1) | (sl->sequence ^ iv); | |
95 | } | |
96 | ||
97 | ||
98 | /* | |
99 | * Version using sequence counter only. | |
100 | * This can be used when code has its own mutex protecting the | |
101 | * updating starting before the write_seqcountbeqin() and ending | |
102 | * after the write_seqcount_end(). | |
103 | */ | |
104 | ||
105 | typedef struct seqcount { | |
106 | unsigned sequence; | |
107 | } seqcount_t; | |
108 | ||
109 | #define SEQCNT_ZERO { 0 } | |
110 | #define seqcount_init(x) do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0) | |
111 | ||
112 | /* Start of read using pointer to a sequence counter only. */ | |
113 | static inline unsigned read_seqcount_begin(const seqcount_t *s) | |
114 | { | |
115 | unsigned ret = s->sequence; | |
116 | smp_rmb(); | |
117 | return ret; | |
118 | } | |
119 | ||
120 | /* Test if reader processed invalid data. | |
121 | * Equivalent to: iv is odd or sequence number has changed. | |
122 | * (iv & 1) || (*s != iv) | |
123 | * Using xor saves one conditional branch. | |
124 | */ | |
125 | static inline int read_seqcount_retry(const seqcount_t *s, unsigned iv) | |
126 | { | |
127 | smp_rmb(); | |
128 | return (iv & 1) | (s->sequence ^ iv); | |
129 | } | |
130 | ||
131 | ||
132 | /* | |
133 | * Sequence counter only version assumes that callers are using their | |
134 | * own mutexing. | |
135 | */ | |
136 | static inline void write_seqcount_begin(seqcount_t *s) | |
137 | { | |
138 | s->sequence++; | |
139 | smp_wmb(); | |
140 | } | |
141 | ||
142 | static inline void write_seqcount_end(seqcount_t *s) | |
143 | { | |
144 | smp_wmb(); | |
145 | s->sequence++; | |
146 | } | |
147 | ||
148 | /* | |
149 | * Possible sw/hw IRQ protected versions of the interfaces. | |
150 | */ | |
151 | #define write_seqlock_irqsave(lock, flags) \ | |
152 | do { local_irq_save(flags); write_seqlock(lock); } while (0) | |
153 | #define write_seqlock_irq(lock) \ | |
154 | do { local_irq_disable(); write_seqlock(lock); } while (0) | |
155 | #define write_seqlock_bh(lock) \ | |
156 | do { local_bh_disable(); write_seqlock(lock); } while (0) | |
157 | ||
158 | #define write_sequnlock_irqrestore(lock, flags) \ | |
159 | do { write_sequnlock(lock); local_irq_restore(flags); } while(0) | |
160 | #define write_sequnlock_irq(lock) \ | |
161 | do { write_sequnlock(lock); local_irq_enable(); } while(0) | |
162 | #define write_sequnlock_bh(lock) \ | |
163 | do { write_sequnlock(lock); local_bh_enable(); } while(0) | |
164 | ||
165 | #define read_seqbegin_irqsave(lock, flags) \ | |
166 | ({ local_irq_save(flags); read_seqbegin(lock); }) | |
167 | ||
168 | #define read_seqretry_irqrestore(lock, iv, flags) \ | |
169 | ({ \ | |
170 | int ret = read_seqretry(lock, iv); \ | |
171 | local_irq_restore(flags); \ | |
172 | ret; \ | |
173 | }) | |
174 | ||
175 | #endif /* __LINUX_SEQLOCK_H */ |