xref: /dragonfly/sys/dev/drm/include/linux/seqlock.h (revision a72de5ad)
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