CRED: Constify the kernel_cap_t arguments to the capset LSM hooks
[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 = 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 */