/include/linux/u64_stats_sync.h

  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 */