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 irq and softirq uses, readers can use u64_stats_fetch_begin_irq() and
  31  *    u64_stats_fetch_retry_irq() 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
  64 struct u64_stats_sync {
  65 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
  66         seqcount_t      seq;
  67 #endif
  68 };
  69
  70
  71 static inline void u64_stats_init(struct u64_stats_sync *syncp)
  72 {
  73 #if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
  74         seqcount_init(&syncp->seq);
  75 #endif
  76 }
  77
  78 static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
  79 {
  80 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
  81         write_seqcount_begin(&syncp->seq);
  82 #endif
  83 }
  84
  85 static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
  86 {
  87 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
  88         write_seqcount_end(&syncp->seq);
  89 #endif
  90 }
  91
  92 static inline void u64_stats_update_begin_raw(struct u64_stats_sync *syncp)
  93 {
  94 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
  95         raw_write_seqcount_begin(&syncp->seq);
  96 #endif
  97 }
  98
  99 static inline void u64_stats_update_end_raw(struct u64_stats_sync *syncp)
 100 {
 101 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 102         raw_write_seqcount_end(&syncp->seq);
 103 #endif
 104 }
 105
 106 static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
 107 {
 108 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 109         return read_seqcount_begin(&syncp->seq);
 110 #else
 111         return 0;
 112 #endif
 113 }
 114
 115 static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
 116 {
 117 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
 118         preempt_disable();
 119 #endif
 120         return __u64_stats_fetch_begin(syncp);
 121 }
 122
 123 static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
 124                                          unsigned int start)
 125 {
 126 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 127         return read_seqcount_retry(&syncp->seq, start);
 128 #else
 129         return false;
 130 #endif
 131 }
 132
 133 static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
 134                                          unsigned int start)
 135 {
 136 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
 137         preempt_enable();
 138 #endif
 139         return __u64_stats_fetch_retry(syncp, start);
 140 }
 141
 142 /*
 143  * In case irq handlers can update u64 counters, readers can use following helpers
 144  * - SMP 32bit arches use seqcount protection, irq safe.
 145  * - UP 32bit must disable irqs.
 146  * - 64bit have no problem atomically reading u64 values, irq safe.
 147  */
 148 static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp)
 149 {
 150 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
 151         local_irq_disable();
 152 #endif
 153         return __u64_stats_fetch_begin(syncp);
 154 }
 155
 156 static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp,
 157                                              unsigned int start)
 158 {
 159 #if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
 160         local_irq_enable();
 161 #endif
 162         return __u64_stats_fetch_retry(syncp, start);
 163 }
 164
 165 #endif /* _LINUX_U64_STATS_SYNC_H */