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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 Dumazet33d91f02010-06-24 00:54:06 +000030 * 7) For softirq uses, readers can use u64_stats_fetch_begin_bh() and
31 * u64_stats_fetch_retry_bh() helpers
32 *
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
71#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
72# define u64_stats_init(syncp) seqcount_init(syncp.seq)
73#else
74# define u64_stats_init(syncp) do { } while (0)
75#endif
76
Jesper Juhlfa9f90b2010-11-28 21:39:34 +010077static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
Eric Dumazet33d91f02010-06-24 00:54:06 +000078{
79#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
80 write_seqcount_begin(&syncp->seq);
81#endif
82}
83
Jesper Juhlfa9f90b2010-11-28 21:39:34 +010084static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
Eric Dumazet33d91f02010-06-24 00:54:06 +000085{
86#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
87 write_seqcount_end(&syncp->seq);
88#endif
89}
90
Jesper Juhlfa9f90b2010-11-28 21:39:34 +010091static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
Eric Dumazet33d91f02010-06-24 00:54:06 +000092{
93#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
94 return read_seqcount_begin(&syncp->seq);
95#else
96#if BITS_PER_LONG==32
97 preempt_disable();
98#endif
99 return 0;
100#endif
101}
102
Jesper Juhlfa9f90b2010-11-28 21:39:34 +0100103static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
Eric Dumazet33d91f02010-06-24 00:54:06 +0000104 unsigned int start)
105{
106#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
107 return read_seqcount_retry(&syncp->seq, start);
108#else
109#if BITS_PER_LONG==32
110 preempt_enable();
111#endif
112 return false;
113#endif
114}
115
116/*
117 * In case softirq handlers can update u64 counters, readers can use following helpers
118 * - SMP 32bit arches use seqcount protection, irq safe.
119 * - UP 32bit must disable BH.
120 * - 64bit have no problem atomically reading u64 values, irq safe.
121 */
Jesper Juhlfa9f90b2010-11-28 21:39:34 +0100122static inline unsigned int u64_stats_fetch_begin_bh(const struct u64_stats_sync *syncp)
Eric Dumazet33d91f02010-06-24 00:54:06 +0000123{
124#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
125 return read_seqcount_begin(&syncp->seq);
126#else
127#if BITS_PER_LONG==32
128 local_bh_disable();
129#endif
130 return 0;
131#endif
132}
133
Jesper Juhlfa9f90b2010-11-28 21:39:34 +0100134static inline bool u64_stats_fetch_retry_bh(const struct u64_stats_sync *syncp,
Eric Dumazet33d91f02010-06-24 00:54:06 +0000135 unsigned int start)
136{
137#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
138 return read_seqcount_retry(&syncp->seq, start);
139#else
140#if BITS_PER_LONG==32
141 local_bh_enable();
142#endif
143 return false;
144#endif
145}
Eric Dumazet16b8a472010-06-22 10:22:17 -0700146
147#endif /* _LINUX_U64_STATS_SYNC_H */