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u64_stats_sync.h
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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 
65 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
67 #endif
68 };
69 
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 
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 
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
91 #endif
92  return 0;
93 #endif
94 }
95 
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  */
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
122 #endif
123  return 0;
124 #endif
125 }
126 
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 */