1/* SPDX-License-Identifier: GPL-2.0 */
2#ifndef _LINUX_U64_STATS_SYNC_H
3#define _LINUX_U64_STATS_SYNC_H
4
5/*
6 * Protect against 64-bit values tearing on 32-bit architectures. This is
7 * typically used for statistics read/update in different subsystems.
8 *
9 * Key points :
10 *
11 * - Use a seqcount on 32-bit
12 * - The whole thing is a no-op on 64-bit architectures.
13 *
14 * Usage constraints:
15 *
16 * 1) Write side must ensure mutual exclusion, or one seqcount update could
17 * be lost, thus blocking readers forever.
18 *
19 * 2) Write side must disable preemption, or a seqcount reader can preempt the
20 * writer and also spin forever.
21 *
22 * 3) Write side must use the _irqsave() variant if other writers, or a reader,
23 * can be invoked from an IRQ context. On 64bit systems this variant does not
24 * disable interrupts.
25 *
26 * 4) If reader fetches several counters, there is no guarantee the whole values
27 * are consistent w.r.t. each other (remember point #2: seqcounts are not
28 * used for 64bit architectures).
29 *
30 * 5) Readers are allowed to sleep or be preempted/interrupted: they perform
31 * pure reads.
32 *
33 * Usage :
34 *
35 * Stats producer (writer) should use following template granted it already got
36 * an exclusive access to counters (a lock is already taken, or per cpu
37 * data is used [in a non preemptable context])
38 *
39 * spin_lock_bh(...) or other synchronization to get exclusive access
40 * ...
41 * u64_stats_update_begin(&stats->syncp);
42 * u64_stats_add(&stats->bytes64, len); // non atomic operation
43 * u64_stats_inc(&stats->packets64); // non atomic operation
44 * u64_stats_update_end(&stats->syncp);
45 *
46 * While a consumer (reader) should use following template to get consistent
47 * snapshot for each variable (but no guarantee on several ones)
48 *
49 * u64 tbytes, tpackets;
50 * unsigned int start;
51 *
52 * do {
53 * start = u64_stats_fetch_begin(&stats->syncp);
54 * tbytes = u64_stats_read(&stats->bytes64); // non atomic operation
55 * tpackets = u64_stats_read(&stats->packets64); // non atomic operation
56 * } while (u64_stats_fetch_retry(&stats->syncp, start));
57 *
58 *
59 * Example of use in drivers/net/loopback.c, using per_cpu containers,
60 * in BH disabled context.
61 */
62#include <linux/seqlock.h>
63
64struct u64_stats_sync {
65#if BITS_PER_LONG == 32
66 seqcount_t seq;
67#endif
68};
69
70#if BITS_PER_LONG == 64
71#include <asm/local64.h>
72
73typedef struct {
74 local64_t v;
75} u64_stats_t ;
76
77static inline u64 u64_stats_read(const u64_stats_t *p)
78{
79 return local64_read(&p->v);
80}
81
82static inline void u64_stats_set(u64_stats_t *p, u64 val)
83{
84 local64_set(&p->v, val);
85}
86
87static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
88{
89 local64_add(val, &p->v);
90}
91
92static inline void u64_stats_inc(u64_stats_t *p)
93{
94 local64_inc(&p->v);
95}
96
97static inline void u64_stats_init(struct u64_stats_sync *syncp) { }
98static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp) { }
99static inline void __u64_stats_update_end(struct u64_stats_sync *syncp) { }
100static inline unsigned long __u64_stats_irqsave(void) { return 0; }
101static inline void __u64_stats_irqrestore(unsigned long flags) { }
102static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
103{
104 return 0;
105}
106static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
107 unsigned int start)
108{
109 return false;
110}
111
112#else /* 64 bit */
113
114typedef struct {
115 u64 v;
116} u64_stats_t;
117
118static inline u64 u64_stats_read(const u64_stats_t *p)
119{
120 return p->v;
121}
122
123static inline void u64_stats_set(u64_stats_t *p, u64 val)
124{
125 p->v = val;
126}
127
128static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
129{
130 p->v += val;
131}
132
133static inline void u64_stats_inc(u64_stats_t *p)
134{
135 p->v++;
136}
137
138static inline void u64_stats_init(struct u64_stats_sync *syncp)
139{
140 seqcount_init(&syncp->seq);
141}
142
143static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp)
144{
145 preempt_disable_nested();
146 write_seqcount_begin(&syncp->seq);
147}
148
149static inline void __u64_stats_update_end(struct u64_stats_sync *syncp)
150{
151 write_seqcount_end(&syncp->seq);
152 preempt_enable_nested();
153}
154
155static inline unsigned long __u64_stats_irqsave(void)
156{
157 unsigned long flags;
158
159 local_irq_save(flags);
160 return flags;
161}
162
163static inline void __u64_stats_irqrestore(unsigned long flags)
164{
165 local_irq_restore(flags);
166}
167
168static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
169{
170 return read_seqcount_begin(&syncp->seq);
171}
172
173static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
174 unsigned int start)
175{
176 return read_seqcount_retry(&syncp->seq, start);
177}
178#endif /* !64 bit */
179
180static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
181{
182 __u64_stats_update_begin(syncp);
183}
184
185static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
186{
187 __u64_stats_update_end(syncp);
188}
189
190static inline unsigned long u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
191{
192 unsigned long flags = __u64_stats_irqsave();
193
194 __u64_stats_update_begin(syncp);
195 return flags;
196}
197
198static inline void u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
199 unsigned long flags)
200{
201 __u64_stats_update_end(syncp);
202 __u64_stats_irqrestore(flags);
203}
204
205static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
206{
207 return __u64_stats_fetch_begin(syncp);
208}
209
210static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
211 unsigned int start)
212{
213 return __u64_stats_fetch_retry(syncp, start);
214}
215
216#endif /* _LINUX_U64_STATS_SYNC_H */
217

source code of linux/include/linux/u64_stats_sync.h