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