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
65struct u64_stats_sync {
66#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
67 seqcount_t seq;
68#endif
69};
70
71
72static 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
79static 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
86static 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
93static inline unsigned long
94u64_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
105static inline void
106u64_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
115static 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
124static 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
132static 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
142static 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 */
157static 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
165static 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