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git.proxmox.com Git - mirror_ubuntu-zesty-kernel.git/blob - include/linux/seqlock.h
1 #ifndef __LINUX_SEQLOCK_H
2 #define __LINUX_SEQLOCK_H
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.
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.
14 * Expected reader usage:
16 * seq = read_seqbegin(&foo);
18 * } while (read_seqretry(&foo, seq));
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.
25 * Based on x86_64 vsyscall gettimeofday
26 * by Keith Owens and Andrea Arcangeli
29 #include <linux/config.h>
30 #include <linux/spinlock.h>
31 #include <linux/preempt.h>
39 * These macros triggered gcc-3.x compile-time problems. We think these are
40 * OK now. Be cautious.
42 #define SEQLOCK_UNLOCKED { 0, SPIN_LOCK_UNLOCKED }
43 #define seqlock_init(x) do { *(x) = (seqlock_t) SEQLOCK_UNLOCKED; } while (0)
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.
50 static inline void write_seqlock(seqlock_t
*sl
)
57 static inline void write_sequnlock(seqlock_t
*sl
)
61 spin_unlock(&sl
->lock
);
64 static inline int write_tryseqlock(seqlock_t
*sl
)
66 int ret
= spin_trylock(&sl
->lock
);
75 /* Start of read calculation -- fetch last complete writer token */
76 static inline unsigned read_seqbegin(const seqlock_t
*sl
)
78 unsigned ret
= sl
->sequence
;
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
89 * Using xor saves one conditional branch.
91 static inline int read_seqretry(const seqlock_t
*sl
, unsigned iv
)
94 return (iv
& 1) | (sl
->sequence
^ iv
);
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().
105 typedef struct seqcount
{
109 #define SEQCNT_ZERO { 0 }
110 #define seqcount_init(x) do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0)
112 /* Start of read using pointer to a sequence counter only. */
113 static inline unsigned read_seqcount_begin(const seqcount_t
*s
)
115 unsigned ret
= s
->sequence
;
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.
125 static inline int read_seqcount_retry(const seqcount_t
*s
, unsigned iv
)
128 return (iv
& 1) | (s
->sequence
^ iv
);
133 * Sequence counter only version assumes that callers are using their
136 static inline void write_seqcount_begin(seqcount_t
*s
)
142 static inline void write_seqcount_end(seqcount_t
*s
)
149 * Possible sw/hw IRQ protected versions of the interfaces.
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)
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)
165 #define read_seqbegin_irqsave(lock, flags) \
166 ({ local_irq_save(flags); read_seqbegin(lock); })
168 #define read_seqretry_irqrestore(lock, iv, flags) \
170 int ret = read_seqretry(lock, iv); \
171 local_irq_restore(flags); \
175 #endif /* __LINUX_SEQLOCK_H */