seqlock.h

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