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  * Usage :
  31  *
  32  * Stats producer (writer) should use following template granted it already got
  33  * an exclusive access to counters (a lock is already taken, or per cpu
  34  * data is used [in a non preemptable context])
  35  *
  36  *   spin_lock_bh(...) or other synchronization to get exclusive access
  37  *   ...
  38  *   u64_stats_update_begin(&stats->syncp);
  39  *   stats->bytes64 += len; // non atomic operation
  40  *   stats->packets64++;    // non atomic operation
  41  *   u64_stats_update_end(&stats->syncp);
  42  *
  43  * While a consumer (reader) should use following template to get consistent
  44  * snapshot for each variable (but no guarantee on several ones)
  45  *
  46  * u64 tbytes, tpackets;
  47  * unsigned int start;
  48  *
  49  * do {
  50  *         start = u64_stats_fetch_begin(&stats->syncp);
  51  *         tbytes = stats->bytes64; // non atomic operation
  52  *         tpackets = stats->packets64; // non atomic operation
  53  * } while (u64_stats_fetch_retry(&stats->syncp, start));
  54  *
  55  *
  56  * Example of use in drivers/net/loopback.c, using per_cpu containers,
  57  * in BH disabled context.
  58  */
  59 #include <linux/seqlock.h>
  60
  61 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
  62 struct u64_stats_sync {
  63         seqcount_t      seq;
  64 };
  65
  66 static void inline u64_stats_update_begin(struct u64_stats_sync *syncp)
  67 {
  68         write_seqcount_begin(&syncp->seq);
  69 }
  70
  71 static void inline u64_stats_update_end(struct u64_stats_sync *syncp)
  72 {
  73         write_seqcount_end(&syncp->seq);
  74 }
  75
  76 static unsigned int inline u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
  77 {
  78         return read_seqcount_begin(&syncp->seq);
  79 }
  80
  81 static bool inline u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
  82                                          unsigned int start)
  83 {
  84         return read_seqcount_retry(&syncp->seq, start);
  85 }
  86
  87 #else
  88 struct u64_stats_sync {
  89 };
  90
  91 static void inline u64_stats_update_begin(struct u64_stats_sync *syncp)
  92 {
  93 }
  94
  95 static void inline u64_stats_update_end(struct u64_stats_sync *syncp)
  96 {
  97 }
  98
  99 static unsigned int inline u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
 100 {
 101         return 0;
 102 }
 103
 104 static bool inline u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
 105                                          unsigned int start)
 106 {
 107         return false;
 108 }
 109 #endif
 110
 111 #endif /* _LINUX_U64_STATS_SYNC_H */