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