blob: fa261a0da280a5e87dd8aece56e66600911fbc1c [file] [log] [blame]
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
70static void inline u64_stats_update_begin(struct u64_stats_sync *syncp)
71{
72#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
73 write_seqcount_begin(&syncp->seq);
74#endif
75}
76
77static void inline u64_stats_update_end(struct u64_stats_sync *syncp)
78{
79#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
80 write_seqcount_end(&syncp->seq);
81#endif
82}
83
84static unsigned int inline u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
85{
86#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
87 return read_seqcount_begin(&syncp->seq);
88#else
89#if BITS_PER_LONG==32
90 preempt_disable();
91#endif
92 return 0;
93#endif
94}
95
96static bool inline u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
97 unsigned int start)
98{
99#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
100 return read_seqcount_retry(&syncp->seq, start);
101#else
102#if BITS_PER_LONG==32
103 preempt_enable();
104#endif
105 return false;
106#endif
107}
108
109/*
110 * In case softirq handlers can update u64 counters, readers can use following helpers
111 * - SMP 32bit arches use seqcount protection, irq safe.
112 * - UP 32bit must disable BH.
113 * - 64bit have no problem atomically reading u64 values, irq safe.
114 */
115static unsigned int inline u64_stats_fetch_begin_bh(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#if BITS_PER_LONG==32
121 local_bh_disable();
122#endif
123 return 0;
124#endif
125}
126
127static bool inline u64_stats_fetch_retry_bh(const struct u64_stats_sync *syncp,
128 unsigned int start)
129{
130#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
131 return read_seqcount_retry(&syncp->seq, start);
132#else
133#if BITS_PER_LONG==32
134 local_bh_enable();
135#endif
136 return false;
137#endif
138}
Eric Dumazet16b8a472010-06-22 10:22:17 -0700139
140#endif /* _LINUX_U64_STATS_SYNC_H */