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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 | * | |
b6b3ecc7 ED |
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 | * | |
16b8a476 ED |
30 | * Usage : |
31 | * | |
32 | * Stats producer (writer) should use following template granted it already got | |
33 | * an exclusive access to counters (a lock is already taken, or per cpu | |
34 | * data is used [in a non preemptable context]) | |
35 | * | |
36 | * spin_lock_bh(...) or other synchronization to get exclusive access | |
37 | * ... | |
38 | * u64_stats_update_begin(&stats->syncp); | |
39 | * stats->bytes64 += len; // non atomic operation | |
40 | * stats->packets64++; // non atomic operation | |
41 | * u64_stats_update_end(&stats->syncp); | |
42 | * | |
43 | * While a consumer (reader) should use following template to get consistent | |
44 | * snapshot for each variable (but no guarantee on several ones) | |
45 | * | |
46 | * u64 tbytes, tpackets; | |
47 | * unsigned int start; | |
48 | * | |
49 | * do { | |
50 | * start = u64_stats_fetch_begin(&stats->syncp); | |
51 | * tbytes = stats->bytes64; // non atomic operation | |
52 | * tpackets = stats->packets64; // non atomic operation | |
b6b3ecc7 | 53 | * } while (u64_stats_fetch_retry(&stats->syncp, start)); |
16b8a476 ED |
54 | * |
55 | * | |
56 | * Example of use in drivers/net/loopback.c, using per_cpu containers, | |
57 | * in BH disabled context. | |
58 | */ | |
59 | #include <linux/seqlock.h> | |
60 | ||
61 | #if BITS_PER_LONG==32 && defined(CONFIG_SMP) | |
62 | struct u64_stats_sync { | |
63 | seqcount_t seq; | |
64 | }; | |
65 | ||
66 | static void inline u64_stats_update_begin(struct u64_stats_sync *syncp) | |
67 | { | |
68 | write_seqcount_begin(&syncp->seq); | |
69 | } | |
70 | ||
71 | static void inline u64_stats_update_end(struct u64_stats_sync *syncp) | |
72 | { | |
73 | write_seqcount_end(&syncp->seq); | |
74 | } | |
75 | ||
76 | static unsigned int inline u64_stats_fetch_begin(const struct u64_stats_sync *syncp) | |
77 | { | |
78 | return read_seqcount_begin(&syncp->seq); | |
79 | } | |
80 | ||
81 | static bool inline u64_stats_fetch_retry(const struct u64_stats_sync *syncp, | |
82 | unsigned int start) | |
83 | { | |
84 | return read_seqcount_retry(&syncp->seq, start); | |
85 | } | |
86 | ||
87 | #else | |
88 | struct u64_stats_sync { | |
89 | }; | |
90 | ||
91 | static void inline u64_stats_update_begin(struct u64_stats_sync *syncp) | |
92 | { | |
93 | } | |
94 | ||
95 | static void inline u64_stats_update_end(struct u64_stats_sync *syncp) | |
96 | { | |
97 | } | |
98 | ||
99 | static unsigned int inline u64_stats_fetch_begin(const struct u64_stats_sync *syncp) | |
100 | { | |
101 | return 0; | |
102 | } | |
103 | ||
104 | static bool inline u64_stats_fetch_retry(const struct u64_stats_sync *syncp, | |
105 | unsigned int start) | |
106 | { | |
107 | return false; | |
108 | } | |
109 | #endif | |
110 | ||
111 | #endif /* _LINUX_U64_STATS_SYNC_H */ |