linux/include/linux/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
  64struct u64_stats_sync {
  65#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
  66        seqcount_t      seq;
  67#endif
  68};
  69
  70static 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
  77static 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
  84static 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
  96static 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 */
 115static 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
 127static 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
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