seqlock/seqlock-wildcard.h

   1 #include <stdatomic.h>
   2 #include <threads.h>
   3
   4 #include "wildcard.h"
   5
   6 typedef struct seqlock {
   7         // Sequence for reader consistency check
   8         atomic_int _seq;
   9         // It needs to be atomic to avoid data races
  10         atomic_int _data;
  11
  12         seqlock() {
  13                 atomic_init(&_seq, 0);
  14                 atomic_init(&_data, 0);
  15         }
  16
  17         int read() {
  18                 while (true) {
  19                         int old_seq = _seq.load(wildcard(1)); // acquire
  20                         if (old_seq % 2 == 1) continue;
  21
  22                         int res = _data.load(wildcard(2)); // acquire
  23                         if (_seq.load(wildcard(3)) == old_seq) { // relaxed
  24                                 return res;
  25                         }
  26                 }
  27         }
  28
  29         void write(int new_data) {
  30                 int old_seq = _seq.load(wildcard(4)); // acquire
  31                 while (true) {
  32                         if (old_seq % 2 == 1)
  33                                 continue; // Retry
  34
  35                         // Should be relaxed!!!
  36                         if (_seq.compare_exchange_strong(old_seq, old_seq + 1,
  37                                 wildcard(5), wildcard(6))) // relaxed
  38                                 break;
  39                 }
  40
  41                 // Update the data
  42                 _data.store(new_data, wildcard(7)); // release
  43
  44                 _seq.fetch_add(1, wildcard(8)); // release
  45         }
  46
  47 } seqlock_t;