1 /* 2 * Copyright (c) 2017 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/lockdep.h> 32 #include <linux/compiler.h> 33 34 typedef struct { 35 unsigned sequence; 36 struct spinlock lock; 37 } seqlock_t; 38 39 40 static inline void 41 seqlock_init(seqlock_t *sl) 42 { 43 sl->sequence = 0; 44 spin_init(&sl->lock, "lsql"); 45 } 46 47 /* 48 * Writers always use a spinlock. We still use store barriers 49 * in order to quickly update the state of the sequence variable 50 * for readers. 51 */ 52 static inline void 53 write_seqlock(seqlock_t *sl) 54 { 55 spin_lock(&sl->lock); 56 sl->sequence++; 57 cpu_sfence(); 58 } 59 60 static inline void 61 write_sequnlock(seqlock_t *sl) 62 { 63 sl->sequence--; 64 spin_unlock(&sl->lock); 65 cpu_sfence(); 66 } 67 68 /* 69 * Read functions are fully unlocked. 70 * We use load barriers to obtain a reasonably up-to-date state 71 * for the sequence number. 72 */ 73 static inline unsigned 74 read_seqbegin(const seqlock_t *sl) 75 { 76 return READ_ONCE(sl->sequence); 77 } 78 79 static inline unsigned 80 read_seqretry(const seqlock_t *sl, unsigned start) 81 { 82 cpu_lfence(); 83 return (sl->sequence != start); 84 } 85 86 #endif /* _LINUX_SEQLOCK_H_ */ 87