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Eric Dumazet16b8a472010-06-22 10:22:17 -07001#ifndef _LINUX_U64_STATS_SYNC_H
2#define _LINUX_U64_STATS_SYNC_H
3
4/*
5 * To properly implement 64bits network statistics on 32bit and 64bit hosts,
6 * we provide a synchronization point, that is a noop on 64bit or UP kernels.
7 *
8 * Key points :
9 * 1) Use a seqcount on SMP 32bits, with low overhead.
10 * 2) Whole thing is a noop on 64bit arches or UP kernels.
11 * 3) Write side must ensure mutual exclusion or one seqcount update could
12 * be lost, thus blocking readers forever.
13 * If this synchronization point is not a mutex, but a spinlock or
14 * spinlock_bh() or disable_bh() :
15 * 3.1) Write side should not sleep.
16 * 3.2) Write side should not allow preemption.
17 * 3.3) If applicable, interrupts should be disabled.
18 *
19 * 4) If reader fetches several counters, there is no guarantee the whole values
20 * are consistent (remember point 1) : this is a noop on 64bit arches anyway)
21 *
22 * 5) readers are allowed to sleep or be preempted/interrupted : They perform
23 * pure reads. But if they have to fetch many values, it's better to not allow
24 * preemptions/interruptions to avoid many retries.
25 *
Eric Dumazetb6b3ecc2010-06-24 00:04:38 +000026 * 6) If counter might be written by an interrupt, readers should block interrupts.
27 * (On UP, there is no seqcount_t protection, a reader allowing interrupts could
28 * read partial values)
29 *
Eric W. Biederman57a77442014-03-13 21:26:42 -070030 * 7) For irq and softirq uses, readers can use u64_stats_fetch_begin_irq() and
31 * u64_stats_fetch_retry_irq() helpers
Eric Dumazet33d91f02010-06-24 00:54:06 +000032 *
Eric Dumazet16b8a472010-06-22 10:22:17 -070033 * 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 * stats->bytes64 += len; // non atomic operation
43 * 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 = stats->bytes64; // non atomic operation
55 * tpackets = stats->packets64; // non atomic operation
Eric Dumazetb6b3ecc2010-06-24 00:04:38 +000056 * } while (u64_stats_fetch_retry(&stats->syncp, start));
Eric Dumazet16b8a472010-06-22 10:22:17 -070057 *
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
Eric Dumazet33d91f02010-06-24 00:54:06 +000064struct u64_stats_sync {
Eric Dumazet16b8a472010-06-22 10:22:17 -070065#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
Eric Dumazet16b8a472010-06-22 10:22:17 -070066 seqcount_t seq;
Eric Dumazet16b8a472010-06-22 10:22:17 -070067#endif
Eric Dumazet33d91f02010-06-24 00:54:06 +000068};
69
John Stultz827da442013-10-07 15:51:58 -070070
Eric Dumazet9464ca62015-06-12 19:44:48 -070071static inline void u64_stats_init(struct u64_stats_sync *syncp)
72{
John Stultz827da442013-10-07 15:51:58 -070073#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
Eric Dumazet9464ca62015-06-12 19:44:48 -070074 seqcount_init(&syncp->seq);
John Stultz827da442013-10-07 15:51:58 -070075#endif
Eric Dumazet9464ca62015-06-12 19:44:48 -070076}
John Stultz827da442013-10-07 15:51:58 -070077
Jesper Juhlfa9f90b2010-11-28 21:39:34 +010078static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
Eric Dumazet33d91f02010-06-24 00:54:06 +000079{
80#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
81 write_seqcount_begin(&syncp->seq);
82#endif
83}
84
Jesper Juhlfa9f90b2010-11-28 21:39:34 +010085static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
Eric Dumazet33d91f02010-06-24 00:54:06 +000086{
87#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
88 write_seqcount_end(&syncp->seq);
89#endif
90}
91
Jesper Juhlfa9f90b2010-11-28 21:39:34 +010092static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
Eric Dumazet33d91f02010-06-24 00:54:06 +000093{
94#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
95 return read_seqcount_begin(&syncp->seq);
96#else
97#if BITS_PER_LONG==32
98 preempt_disable();
99#endif
100 return 0;
101#endif
102}
103
Jesper Juhlfa9f90b2010-11-28 21:39:34 +0100104static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
Eric Dumazet33d91f02010-06-24 00:54:06 +0000105 unsigned int start)
106{
107#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
108 return read_seqcount_retry(&syncp->seq, start);
109#else
110#if BITS_PER_LONG==32
111 preempt_enable();
112#endif
113 return false;
114#endif
115}
116
117/*
Eric W. Biederman57a77442014-03-13 21:26:42 -0700118 * In case irq handlers can update u64 counters, readers can use following helpers
Eric Dumazet33d91f02010-06-24 00:54:06 +0000119 * - SMP 32bit arches use seqcount protection, irq safe.
Eric W. Biederman57a77442014-03-13 21:26:42 -0700120 * - UP 32bit must disable irqs.
Eric Dumazet33d91f02010-06-24 00:54:06 +0000121 * - 64bit have no problem atomically reading u64 values, irq safe.
122 */
Eric W. Biederman57a77442014-03-13 21:26:42 -0700123static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp)
Eric Dumazet33d91f02010-06-24 00:54:06 +0000124{
125#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
126 return read_seqcount_begin(&syncp->seq);
127#else
128#if BITS_PER_LONG==32
Eric W. Biederman57a77442014-03-13 21:26:42 -0700129 local_irq_disable();
Eric Dumazet33d91f02010-06-24 00:54:06 +0000130#endif
131 return 0;
132#endif
133}
134
Eric W. Biederman57a77442014-03-13 21:26:42 -0700135static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp,
Eric Dumazet33d91f02010-06-24 00:54:06 +0000136 unsigned int start)
137{
138#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
139 return read_seqcount_retry(&syncp->seq, start);
140#else
141#if BITS_PER_LONG==32
Eric W. Biederman57a77442014-03-13 21:26:42 -0700142 local_irq_enable();
Eric Dumazet33d91f02010-06-24 00:54:06 +0000143#endif
144 return false;
145#endif
146}
Eric Dumazet16b8a472010-06-22 10:22:17 -0700147
148#endif /* _LINUX_U64_STATS_SYNC_H */