1 #ifndef _LINUX_U64_STATS_SYNC_H
2 #define _LINUX_U64_STATS_SYNC_H
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.
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.
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)
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.
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)
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])
36 * spin_lock_bh(...) or other synchronization to get exclusive access
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);
43 * While a consumer (reader) should use following template to get consistent
44 * snapshot for each variable (but no guarantee on several ones)
46 * u64 tbytes, tpackets;
50 * start = u64_stats_fetch_begin(&stats->syncp);
51 * tbytes = stats->bytes64; // non atomic operation
52 * tpackets = stats->packets64; // non atomic operation
53 * } while (u64_stats_fetch_retry(&stats->syncp, start));
56 * Example of use in drivers/net/loopback.c, using per_cpu containers,
57 * in BH disabled context.
59 #include <linux/seqlock.h>
61 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
62 struct u64_stats_sync {
66 static void inline u64_stats_update_begin(struct u64_stats_sync *syncp)
68 write_seqcount_begin(&syncp->seq);
71 static void inline u64_stats_update_end(struct u64_stats_sync *syncp)
73 write_seqcount_end(&syncp->seq);
76 static unsigned int inline u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
78 return read_seqcount_begin(&syncp->seq);
81 static bool inline u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
84 return read_seqcount_retry(&syncp->seq, start);
88 struct u64_stats_sync {
91 static void inline u64_stats_update_begin(struct u64_stats_sync *syncp)
95 static void inline u64_stats_update_end(struct u64_stats_sync *syncp)
99 static unsigned int inline u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
104 static bool inline u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
111 #endif /* _LINUX_U64_STATS_SYNC_H */