LXR linux/include/linux/u64_stats

   1/* SPDX-License-Identifier: GPL-2.0 */
   2#ifndef _LINUX_U64_STATS_SYNC_H
   3#define _LINUX_U64_STATS_SYNC_H
   4
   5/*
   6 * To properly implement 64bits network statistics on 32bit and 64bit hosts,
   7 * we provide a synchronization point, that is a noop on 64bit or UP kernels.
   8 *
   9 * Key points :
  10 * 1) Use a seqcount on SMP 32bits, with low overhead.
  11 * 2) Whole thing is a noop on 64bit arches or UP kernels.
  12 * 3) Write side must ensure mutual exclusion or one seqcount update could
  13 *    be lost, thus blocking readers forever.
  14 *    If this synchronization point is not a mutex, but a spinlock or
  15 *    spinlock_bh() or disable_bh() :
  16 * 3.1) Write side should not sleep.
  17 * 3.2) Write side should not allow preemption.
  18 * 3.3) If applicable, interrupts should be disabled.
  19 *
  20 * 4) If reader fetches several counters, there is no guarantee the whole values
  21 *    are consistent (remember point 1) : this is a noop on 64bit arches anyway)
  22 *
  23 * 5) readers are allowed to sleep or be preempted/interrupted : They perform
  24 *    pure reads. But if they have to fetch many values, it's better to not allow
  25 *    preemptions/interruptions to avoid many retries.
  26 *
  27 * 6) If counter might be written by an interrupt, readers should block interrupts.
  28 *    (On UP, there is no seqcount_t protection, a reader allowing interrupts could
  29 *     read partial values)
  30 *
  31 * 7) For irq and softirq uses, readers can use u64_stats_fetch_begin_irq() and
  32 *    u64_stats_fetch_retry_irq() helpers
  33 *
  34 * Usage :
  35 *
  36 * Stats producer (writer) should use following template granted it already got
  37 * an exclusive access to counters (a lock is already taken, or per cpu
  38 * data is used [in a non preemptable context])
  39 *
  40 *   spin_lock_bh(...) or other synchronization to get exclusive access
  41 *   ...
  42 *   u64_stats_update_begin(&stats->syncp);
  43 *   stats->bytes64 += len; // non atomic operation
  44 *   stats->packets64++;    // non atomic operation
  45 *   u64_stats_update_end(&stats->syncp);
  46 *
  47 * While a consumer (reader) should use following template to get consistent
  48 * snapshot for each variable (but no guarantee on several ones)
  49 *
  50 * u64 tbytes, tpackets;
  51 * unsigned int start;
  52 *
  53 * do {
  54 *         start = u64_stats_fetch_begin(&stats->syncp);
  55 *         tbytes = stats->bytes64; // non atomic operation
  56 *         tpackets = stats->packets64; // non atomic operation
  57 * } while (u64_stats_fetch_retry(&stats->syncp, start));
  58 *
  59 *
  60 * Example of use in drivers/net/loopback.c, using per_cpu containers,
  61 * in BH disabled context.
  62 */
  63#include <linux/seqlock.h>
  64
  65struct u64_stats_sync {
  66#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
  67        seqcount_t      seq;
  68#endif
  69};
  70
  71
  72static inline void u64_stats_init(struct u64_stats_sync *syncp)
  73{
  74#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
  75        seqcount_init(&syncp->seq);
  76#endif
  77}
  78
  79static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
  80{
  81#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
  82        write_seqcount_begin(&syncp->seq);
  83#endif
  84}
  85
  86static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
  87{
  88#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
  89        write_seqcount_end(&syncp->seq);
  90#endif
  91}
  92
  93static inline void u64_stats_update_begin_raw(struct u64_stats_sync *syncp)
  94{
  95#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
  96        raw_write_seqcount_begin(&syncp->seq);
  97#endif
  98}
  99
 100static inline void u64_stats_update_end_raw(struct u64_stats_sync *syncp)
 101{
 102#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 103        raw_write_seqcount_end(&syncp->seq);
 104#endif
 105}
 106
 107static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
 108{
 109#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 110        return read_seqcount_begin(&syncp->seq);
 111#else
 112        return 0;
 113#endif
 114}
 115
 116static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
 117{
 118#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
 119        preempt_disable();
 120#endif
 121        return __u64_stats_fetch_begin(syncp);
 122}
 123
 124static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
 125                                         unsigned int start)
 126{
 127#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 128        return read_seqcount_retry(&syncp->seq, start);
 129#else
 130        return false;
 131#endif
 132}
 133
 134static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
 135                                         unsigned int start)
 136{
 137#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
 138        preempt_enable();
 139#endif
 140        return __u64_stats_fetch_retry(syncp, start);
 141}
 142
 143/*
 144 * In case irq handlers can update u64 counters, readers can use following helpers
 145 * - SMP 32bit arches use seqcount protection, irq safe.
 146 * - UP 32bit must disable irqs.
 147 * - 64bit have no problem atomically reading u64 values, irq safe.
 148 */
 149static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp)
 150{
 151#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
 152        local_irq_disable();
 153#endif
 154        return __u64_stats_fetch_begin(syncp);
 155}
 156
 157static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp,
 158                                             unsigned int start)
 159{
 160#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
 161        local_irq_enable();
 162#endif
 163        return __u64_stats_fetch_retry(syncp, start);
 164}
 165
 166#endif /* _LINUX_U64_STATS_SYNC_H */
 167