1 #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 *
26 * 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 *
30 * 7) For softirq uses, readers can use u64_stats_fetch_begin_bh() and
31 * u64_stats_fetch_retry_bh() helpers
32 *
33 * 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
56 * } while (u64_stats_fetch_retry(&stats->syncp, start));
57 *
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
64 struct u64_stats_sync {
65 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
66 seqcount_t seq;
67 #endif
68 };
69
u64_stats_update_begin(struct u64_stats_sync * syncp)70 static inline void 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
u64_stats_update_end(struct u64_stats_sync * syncp)77 static inline void 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
u64_stats_fetch_begin(const struct u64_stats_sync * syncp)84 static inline unsigned int 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
u64_stats_fetch_retry(const struct u64_stats_sync * syncp,unsigned int start)96 static inline bool 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 */
u64_stats_fetch_begin_bh(const struct u64_stats_sync * syncp)115 static inline unsigned int 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
u64_stats_fetch_retry_bh(const struct u64_stats_sync * syncp,unsigned int start)127 static inline bool 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 }
139
140 #endif /* _LINUX_U64_STATS_SYNC_H */
141