1 /* 2 * Copyright (c) 2017-2020 François Tigeot <ftigeot@wolfpond.org> 3 * All rights reserved. 4 * 5 * Redistribution and use in source and binary forms, with or without 6 * modification, are permitted provided that the following conditions 7 * are met: 8 * 1. Redistributions of source code must retain the above copyright 9 * notice unmodified, this list of conditions, and the following 10 * disclaimer. 11 * 2. Redistributions in binary form must reproduce the above copyright 12 * notice, this list of conditions and the following disclaimer in the 13 * documentation and/or other materials provided with the distribution. 14 * 15 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 16 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 17 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 18 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 19 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 20 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 21 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 22 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 23 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 24 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 25 */ 26 27 #ifndef _LINUX_SEQLOCK_H_ 28 #define _LINUX_SEQLOCK_H_ 29 30 #include <linux/spinlock.h> 31 #include <linux/preempt.h> 32 #include <linux/lockdep.h> 33 #include <linux/compiler.h> 34 #include <asm/processor.h> 35 36 /* 37 * Seqlock definition from Wikipedia 38 * 39 * A seqlock consists of storage for saving a sequence number in addition to a lock. 40 * The lock is to support synchronization between two writers and the counter is for 41 * indicating consistency in readers. In addition to updating the shared data, the 42 * writer increments the sequence number, both after acquiring the lock and before 43 * releasing the lock. Readers read the sequence number before and after reading the 44 * shared data. If the sequence number is odd on either occasion, a writer had taken 45 * the lock while the data was being read and it may have changed. If the sequence 46 * numbers are different, a writer has changed the data while it was being read. In 47 * either case readers simply retry (using a loop) until they read the same even 48 * sequence number before and after. 49 * 50 */ 51 52 typedef struct { 53 unsigned sequence; 54 struct lock lock; 55 } seqlock_t; 56 57 static inline void 58 seqlock_init(seqlock_t *sl) 59 { 60 sl->sequence = 0; 61 lockinit(&sl->lock, "lsql", 0, 0); 62 } 63 64 /* 65 * Writers always use a spinlock. We still use store barriers 66 * in order to quickly update the state of the sequence variable 67 * for readers. 68 */ 69 static inline void 70 write_seqlock(seqlock_t *sl) 71 { 72 lockmgr(&sl->lock, LK_EXCLUSIVE); 73 sl->sequence++; 74 cpu_sfence(); 75 } 76 77 static inline void 78 write_sequnlock(seqlock_t *sl) 79 { 80 sl->sequence--; 81 lockmgr(&sl->lock, LK_RELEASE); 82 cpu_sfence(); 83 } 84 85 /* 86 * Read functions are fully unlocked. 87 * We use load barriers to obtain a reasonably up-to-date state 88 * for the sequence number. 89 */ 90 static inline unsigned 91 read_seqbegin(const seqlock_t *sl) 92 { 93 return READ_ONCE(sl->sequence); 94 } 95 96 static inline unsigned 97 read_seqretry(const seqlock_t *sl, unsigned start) 98 { 99 cpu_lfence(); 100 return (sl->sequence != start); 101 } 102 103 typedef struct seqcount { 104 unsigned sequence; 105 } seqcount_t; 106 107 static inline void 108 __seqcount_init(seqcount_t *s, const char *name, struct lock_class_key *key) 109 { 110 s->sequence = 0; 111 } 112 113 static inline unsigned int 114 read_seqcount_begin(const seqcount_t *s) 115 { 116 unsigned int ret; 117 118 do { 119 ret = READ_ONCE(s->sequence); 120 /* If the sequence number is odd, a writer has taken the lock */ 121 if ((ret & 1) == 0) 122 break; 123 cpu_pause(); 124 } while (1); 125 126 cpu_lfence(); 127 return ret; 128 } 129 130 static inline int read_seqcount_retry(const seqcount_t *s, unsigned start) 131 { 132 cpu_lfence(); 133 return (s->sequence != start); 134 } 135 136 static inline void write_seqcount_begin(seqcount_t *s) 137 { 138 s->sequence++; 139 cpu_ccfence(); 140 } 141 142 static inline void write_seqcount_end(seqcount_t *s) 143 { 144 cpu_ccfence(); 145 s->sequence++; 146 } 147 148 #endif /* _LINUX_SEQLOCK_H_ */ 149