seqlock/seqlock.h

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