1 /* 2 * Copyright (c) 2017-2019 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 typedef struct { 37 unsigned sequence; 38 struct spinlock lock; 39 } seqlock_t; 40 41 static inline void 42 seqlock_init(seqlock_t *sl) 43 { 44 sl->sequence = 0; 45 spin_init(&sl->lock, "lsql"); 46 } 47 48 /* 49 * Writers always use a spinlock. We still use store barriers 50 * in order to quickly update the state of the sequence variable 51 * for readers. 52 */ 53 static inline void 54 write_seqlock(seqlock_t *sl) 55 { 56 spin_lock(&sl->lock); 57 sl->sequence++; 58 cpu_sfence(); 59 } 60 61 static inline void 62 write_sequnlock(seqlock_t *sl) 63 { 64 sl->sequence--; 65 spin_unlock(&sl->lock); 66 cpu_sfence(); 67 } 68 69 /* 70 * Read functions are fully unlocked. 71 * We use load barriers to obtain a reasonably up-to-date state 72 * for the sequence number. 73 */ 74 static inline unsigned 75 read_seqbegin(const seqlock_t *sl) 76 { 77 return READ_ONCE(sl->sequence); 78 } 79 80 static inline unsigned 81 read_seqretry(const seqlock_t *sl, unsigned start) 82 { 83 cpu_lfence(); 84 return (sl->sequence != start); 85 } 86 87 typedef struct seqcount { 88 unsigned sequence; 89 } seqcount_t; 90 91 static inline void 92 __seqcount_init(seqcount_t *s, const char *name, struct lock_class_key *key) 93 { 94 s->sequence = 0; 95 } 96 97 #endif /* _LINUX_SEQLOCK_H_ */ 98