user namespace: add the framework
[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 = sl->sequence;
89         smp_rmb();
90         return ret;
91 }
92
93 /* Test if reader processed invalid data.
94  * If initial values is odd, 
95  *      then writer had already started when section was entered
96  * If sequence value changed
97  *      then writer changed data while in section
98  *    
99  * Using xor saves one conditional branch.
100  */
101 static __always_inline int read_seqretry(const seqlock_t *sl, unsigned iv)
102 {
103         smp_rmb();
104         return (iv & 1) | (sl->sequence ^ iv);
105 }
106
107
108 /*
109  * Version using sequence counter only.
110  * This can be used when code has its own mutex protecting the
111  * updating starting before the write_seqcountbeqin() and ending
112  * after the write_seqcount_end().
113  */
114
115 typedef struct seqcount {
116         unsigned sequence;
117 } seqcount_t;
118
119 #define SEQCNT_ZERO { 0 }
120 #define seqcount_init(x)        do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0)
121
122 /* Start of read using pointer to a sequence counter only.  */
123 static inline unsigned read_seqcount_begin(const seqcount_t *s)
124 {
125         unsigned ret = s->sequence;
126         smp_rmb();
127         return ret;
128 }
129
130 /* Test if reader processed invalid data.
131  * Equivalent to: iv is odd or sequence number has changed.
132  *                (iv & 1) || (*s != iv)
133  * Using xor saves one conditional branch.
134  */
135 static inline int read_seqcount_retry(const seqcount_t *s, unsigned iv)
136 {
137         smp_rmb();
138         return (iv & 1) | (s->sequence ^ iv);
139 }
140
141
142 /*
143  * Sequence counter only version assumes that callers are using their
144  * own mutexing.
145  */
146 static inline void write_seqcount_begin(seqcount_t *s)
147 {
148         s->sequence++;
149         smp_wmb();
150 }
151
152 static inline void write_seqcount_end(seqcount_t *s)
153 {
154         smp_wmb();
155         s->sequence++;
156 }
157
158 /*
159  * Possible sw/hw IRQ protected versions of the interfaces.
160  */
161 #define write_seqlock_irqsave(lock, flags)                              \
162         do { local_irq_save(flags); write_seqlock(lock); } while (0)
163 #define write_seqlock_irq(lock)                                         \
164         do { local_irq_disable();   write_seqlock(lock); } while (0)
165 #define write_seqlock_bh(lock)                                          \
166         do { local_bh_disable();    write_seqlock(lock); } while (0)
167
168 #define write_sequnlock_irqrestore(lock, flags)                         \
169         do { write_sequnlock(lock); local_irq_restore(flags); } while(0)
170 #define write_sequnlock_irq(lock)                                       \
171         do { write_sequnlock(lock); local_irq_enable(); } while(0)
172 #define write_sequnlock_bh(lock)                                        \
173         do { write_sequnlock(lock); local_bh_enable(); } while(0)
174
175 #define read_seqbegin_irqsave(lock, flags)                              \
176         ({ local_irq_save(flags);   read_seqbegin(lock); })
177
178 #define read_seqretry_irqrestore(lock, iv, flags)                       \
179         ({                                                              \
180                 int ret = read_seqretry(lock, iv);                      \
181                 local_irq_restore(flags);                               \
182                 ret;                                                    \
183         })
184
185 #endif /* __LINUX_SEQLOCK_H */