1 /* SPDX-License-Identifier: GPL-2.0 */
2 #ifndef _LINUX_U64_STATS_SYNC_H
3 #define _LINUX_U64_STATS_SYNC_H
4
5 /*
6 * To properly implement 64bits network statistics on 32bit and 64bit hosts,
7 * we provide a synchronization point, that is a noop on 64bit or UP kernels.
8 *
9 * Key points :
10 * 1) Use a seqcount on SMP 32bits, with low overhead.
11 * 2) Whole thing is a noop on 64bit arches or UP kernels.
12 * 3) Write side must ensure mutual exclusion or one seqcount update could
13 * be lost, thus blocking readers forever.
14 * If this synchronization point is not a mutex, but a spinlock or
15 * spinlock_bh() or disable_bh() :
16 * 3.1) Write side should not sleep.
17 * 3.2) Write side should not allow preemption.
18 * 3.3) If applicable, interrupts should be disabled.
19 *
20 * 4) If reader fetches several counters, there is no guarantee the whole values
21 * are consistent (remember point 1) : this is a noop on 64bit arches anyway)
22 *
23 * 5) readers are allowed to sleep or be preempted/interrupted : They perform
24 * pure reads. But if they have to fetch many values, it's better to not allow
25 * preemptions/interruptions to avoid many retries.
26 *
27 * 6) If counter might be written by an interrupt, readers should block interrupts.
28 * (On UP, there is no seqcount_t protection, a reader allowing interrupts could
29 * read partial values)
30 *
31 * 7) For irq and softirq uses, readers can use u64_stats_fetch_begin_irq() and
32 * u64_stats_fetch_retry_irq() helpers
33 *
34 * Usage :
35 *
36 * Stats producer (writer) should use following template granted it already got
37 * an exclusive access to counters (a lock is already taken, or per cpu
38 * data is used [in a non preemptable context])
39 *
40 * spin_lock_bh(...) or other synchronization to get exclusive access
41 * ...
42 * u64_stats_update_begin(&stats->syncp);
43 * stats->bytes64 += len; // non atomic operation
44 * stats->packets64++; // non atomic operation
45 * u64_stats_update_end(&stats->syncp);
46 *
47 * While a consumer (reader) should use following template to get consistent
48 * snapshot for each variable (but no guarantee on several ones)
49 *
50 * u64 tbytes, tpackets;
51 * unsigned int start;
52 *
53 * do {
54 * start = u64_stats_fetch_begin(&stats->syncp);
55 * tbytes = stats->bytes64; // non atomic operation
56 * tpackets = stats->packets64; // non atomic operation
57 * } while (u64_stats_fetch_retry(&stats->syncp, start));
58 *
59 *
60 * Example of use in drivers/net/loopback.c, using per_cpu containers,
61 * in BH disabled context.
62 */
63 #include <linux/seqlock.h>
64
65 struct u64_stats_sync {
66 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
67 seqcount_t seq;
68 #endif
69 };
70
71
u64_stats_init(struct u64_stats_sync * syncp)72 static inline void u64_stats_init(struct u64_stats_sync *syncp)
73 {
74 #if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
75 seqcount_init(&syncp->seq);
76 #endif
77 }
78
u64_stats_update_begin(struct u64_stats_sync * syncp)79 static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
80 {
81 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
82 write_seqcount_begin(&syncp->seq);
83 #endif
84 }
85
u64_stats_update_end(struct u64_stats_sync * syncp)86 static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
87 {
88 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
89 write_seqcount_end(&syncp->seq);
90 #endif
91 }
92
93 static inline unsigned long
u64_stats_update_begin_irqsave(struct u64_stats_sync * syncp)94 u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
95 {
96 unsigned long flags = 0;
97
98 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
99 local_irq_save(flags);
100 write_seqcount_begin(&syncp->seq);
101 #endif
102 return flags;
103 }
104
105 static inline void
u64_stats_update_end_irqrestore(struct u64_stats_sync * syncp,unsigned long flags)106 u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
107 unsigned long flags)
108 {
109 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
110 write_seqcount_end(&syncp->seq);
111 local_irq_restore(flags);
112 #endif
113 }
114
__u64_stats_fetch_begin(const struct u64_stats_sync * syncp)115 static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
116 {
117 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
118 return read_seqcount_begin(&syncp->seq);
119 #else
120 return 0;
121 #endif
122 }
123
u64_stats_fetch_begin(const struct u64_stats_sync * syncp)124 static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
125 {
126 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
127 preempt_disable();
128 #endif
129 return __u64_stats_fetch_begin(syncp);
130 }
131
__u64_stats_fetch_retry(const struct u64_stats_sync * syncp,unsigned int start)132 static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
133 unsigned int start)
134 {
135 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
136 return read_seqcount_retry(&syncp->seq, start);
137 #else
138 return false;
139 #endif
140 }
141
u64_stats_fetch_retry(const struct u64_stats_sync * syncp,unsigned int start)142 static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
143 unsigned int start)
144 {
145 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
146 preempt_enable();
147 #endif
148 return __u64_stats_fetch_retry(syncp, start);
149 }
150
151 /*
152 * In case irq handlers can update u64 counters, readers can use following helpers
153 * - SMP 32bit arches use seqcount protection, irq safe.
154 * - UP 32bit must disable irqs.
155 * - 64bit have no problem atomically reading u64 values, irq safe.
156 */
u64_stats_fetch_begin_irq(const struct u64_stats_sync * syncp)157 static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp)
158 {
159 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
160 local_irq_disable();
161 #endif
162 return __u64_stats_fetch_begin(syncp);
163 }
164
u64_stats_fetch_retry_irq(const struct u64_stats_sync * syncp,unsigned int start)165 static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp,
166 unsigned int start)
167 {
168 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
169 local_irq_enable();
170 #endif
171 return __u64_stats_fetch_retry(syncp, start);
172 }
173
174 #endif /* _LINUX_U64_STATS_SYNC_H */
175
