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
64 struct 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
73 typedef struct {
74 local64_t v;
75 } u64_stats_t ;
76
77 static inline u64 u64_stats_read(const u64_stats_t *p)
78 {
79 return local64_read(&p->v);
80 }
81
82 static inline void u64_stats_set(u64_stats_t *p, u64 val)
83 {
84 local64_set(&p->v, val);
85 }
86
87 static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
88 {
89 local64_add(val, &p->v);
90 }
91
92 static inline void u64_stats_inc(u64_stats_t *p)
93 {
94 local64_inc(&p->v);
95 }
96
97 static inline void u64_stats_init(struct u64_stats_sync *syncp) { }
98 static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp) { }
99 static inline void __u64_stats_update_end(struct u64_stats_sync *syncp) { }
100 static inline unsigned long __u64_stats_irqsave(void) { return 0; }
101 static inline void __u64_stats_irqrestore(unsigned long flags) { }
102 static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
103 {
104 return 0;
105 }
106 static 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
114 typedef struct {
115 u64 v;
116 } u64_stats_t;
117
118 static inline u64 u64_stats_read(const u64_stats_t *p)
119 {
120 return p->v;
121 }
122
123 static inline void u64_stats_set(u64_stats_t *p, u64 val)
124 {
125 p->v = val;
126 }
127
128 static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
129 {
130 p->v += val;
131 }
132
133 static inline void u64_stats_inc(u64_stats_t *p)
134 {
135 p->v++;
136 }
137
138 #define u64_stats_init(syncp) \
139 do { \
140 struct u64_stats_sync *__s = (syncp); \
141 seqcount_init(&__s->seq); \
142 } while (0)
143
144 static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp)
145 {
146 preempt_disable_nested();
147 write_seqcount_begin(&syncp->seq);
148 }
149
150 static inline void __u64_stats_update_end(struct u64_stats_sync *syncp)
151 {
152 write_seqcount_end(&syncp->seq);
153 preempt_enable_nested();
154 }
155
156 static inline unsigned long __u64_stats_irqsave(void)
157 {
158 unsigned long flags;
159
160 local_irq_save(flags);
161 return flags;
162 }
163
164 static inline void __u64_stats_irqrestore(unsigned long flags)
165 {
166 local_irq_restore(flags);
167 }
168
169 static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
170 {
171 return read_seqcount_begin(&syncp->seq);
172 }
173
174 static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
175 unsigned int start)
176 {
177 return read_seqcount_retry(&syncp->seq, start);
178 }
179 #endif /* !64 bit */
180
181 static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
182 {
183 __u64_stats_update_begin(syncp);
184 }
185
186 static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
187 {
188 __u64_stats_update_end(syncp);
189 }
190
191 static inline unsigned long u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
192 {
193 unsigned long flags = __u64_stats_irqsave();
194
195 __u64_stats_update_begin(syncp);
196 return flags;
197 }
198
199 static inline void u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
200 unsigned long flags)
201 {
202 __u64_stats_update_end(syncp);
203 __u64_stats_irqrestore(flags);
204 }
205
206 static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
207 {
208 return __u64_stats_fetch_begin(syncp);
209 }
210
211 static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
212 unsigned int start)
213 {
214 return __u64_stats_fetch_retry(syncp, start);
215 }
216
217 #endif /* _LINUX_U64_STATS_SYNC_H */
218
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