1 #ifndef JEMALLOC_INTERNAL_SEQ_H
2 #define JEMALLOC_INTERNAL_SEQ_H
3 
4 #include "jemalloc/internal/atomic.h"
5 
6 /*
7  * A simple seqlock implementation.
8  */
9 
10 #define seq_define(type, short_type)					\
11 typedef struct {							\
12 	atomic_zu_t seq;						\
13 	atomic_zu_t data[						\
14 	    (sizeof(type) + sizeof(size_t) - 1) / sizeof(size_t)];	\
15 } seq_##short_type##_t;							\
16 									\
17 /*									\
18  * No internal synchronization -- the caller must ensure that there's	\
19  * only a single writer at a time.					\
20  */									\
21 static inline void							\
22 seq_store_##short_type(seq_##short_type##_t *dst, type *src) {		\
23 	size_t buf[sizeof(dst->data) / sizeof(size_t)];			\
24 	buf[sizeof(buf) / sizeof(size_t) - 1] = 0;			\
25 	memcpy(buf, src, sizeof(type));					\
26 	size_t old_seq = atomic_load_zu(&dst->seq, ATOMIC_RELAXED);	\
27 	atomic_store_zu(&dst->seq, old_seq + 1, ATOMIC_RELAXED);	\
28 	atomic_fence(ATOMIC_RELEASE);					\
29 	for (size_t i = 0; i < sizeof(buf) / sizeof(size_t); i++) {	\
30 		atomic_store_zu(&dst->data[i], buf[i], ATOMIC_RELAXED);	\
31 	}								\
32 	atomic_store_zu(&dst->seq, old_seq + 2, ATOMIC_RELEASE);	\
33 }									\
34 									\
35 /* Returns whether or not the read was consistent. */			\
36 static inline bool							\
37 seq_try_load_##short_type(type *dst, seq_##short_type##_t *src) {	\
38 	size_t buf[sizeof(src->data) / sizeof(size_t)];			\
39 	size_t seq1 = atomic_load_zu(&src->seq, ATOMIC_ACQUIRE);	\
40 	if (seq1 % 2 != 0) {						\
41 		return false;						\
42 	}								\
43 	for (size_t i = 0; i < sizeof(buf) / sizeof(size_t); i++) {	\
44 		buf[i] = atomic_load_zu(&src->data[i], ATOMIC_RELAXED);	\
45 	}								\
46 	atomic_fence(ATOMIC_ACQUIRE);					\
47 	size_t seq2 = atomic_load_zu(&src->seq, ATOMIC_RELAXED);	\
48 	if (seq1 != seq2) {						\
49 		return false;						\
50 	}								\
51 	memcpy(dst, buf, sizeof(type));					\
52 	return true;							\
53 }
54 
55 #endif /* JEMALLOC_INTERNAL_SEQ_H */
56