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
72 #if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
73 #define u64_stats_init(syncp) seqcount_init(&(syncp)->seq)
74 #else
u64_stats_init(struct u64_stats_sync * syncp)75 static inline void u64_stats_init(struct u64_stats_sync *syncp)
76 {
77 }
78 #endif
79
u64_stats_update_begin(struct u64_stats_sync * syncp)80 static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
81 {
82 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
83 write_seqcount_begin(&syncp->seq);
84 #endif
85 }
86
u64_stats_update_end(struct u64_stats_sync * syncp)87 static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
88 {
89 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
90 write_seqcount_end(&syncp->seq);
91 #endif
92 }
93
94 static inline unsigned long
u64_stats_update_begin_irqsave(struct u64_stats_sync * syncp)95 u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
96 {
97 unsigned long flags = 0;
98
99 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
100 local_irq_save(flags);
101 write_seqcount_begin(&syncp->seq);
102 #endif
103 return flags;
104 }
105
106 static inline void
u64_stats_update_end_irqrestore(struct u64_stats_sync * syncp,unsigned long flags)107 u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
108 unsigned long flags)
109 {
110 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
111 write_seqcount_end(&syncp->seq);
112 local_irq_restore(flags);
113 #endif
114 }
115
__u64_stats_fetch_begin(const struct u64_stats_sync * syncp)116 static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
117 {
118 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
119 return read_seqcount_begin(&syncp->seq);
120 #else
121 return 0;
122 #endif
123 }
124
u64_stats_fetch_begin(const struct u64_stats_sync * syncp)125 static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
126 {
127 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
128 preempt_disable();
129 #endif
130 return __u64_stats_fetch_begin(syncp);
131 }
132
__u64_stats_fetch_retry(const struct u64_stats_sync * syncp,unsigned int start)133 static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
134 unsigned int start)
135 {
136 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
137 return read_seqcount_retry(&syncp->seq, start);
138 #else
139 return false;
140 #endif
141 }
142
u64_stats_fetch_retry(const struct u64_stats_sync * syncp,unsigned int start)143 static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
144 unsigned int start)
145 {
146 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
147 preempt_enable();
148 #endif
149 return __u64_stats_fetch_retry(syncp, start);
150 }
151
152 /*
153 * In case irq handlers can update u64 counters, readers can use following helpers
154 * - SMP 32bit arches use seqcount protection, irq safe.
155 * - UP 32bit must disable irqs.
156 * - 64bit have no problem atomically reading u64 values, irq safe.
157 */
u64_stats_fetch_begin_irq(const struct u64_stats_sync * syncp)158 static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp)
159 {
160 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
161 local_irq_disable();
162 #endif
163 return __u64_stats_fetch_begin(syncp);
164 }
165
u64_stats_fetch_retry_irq(const struct u64_stats_sync * syncp,unsigned int start)166 static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp,
167 unsigned int start)
168 {
169 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
170 local_irq_enable();
171 #endif
172 return __u64_stats_fetch_retry(syncp, start);
173 }
174
175 #endif /* _LINUX_U64_STATS_SYNC_H */
176