seqlock/seqlock.c

   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;
  47
  48
  49 seqlock_t *lock;
  50
  51 static void a(void *obj) {
  52         lock->write(3);
  53 }
  54
  55 static void b(void *obj) {
  56         lock->write(2);
  57 }
  58
  59 static void c(void *obj) {
  60         int r1 = lock->read();
  61 }
  62
  63 int user_main(int argc, char **argv) {
  64         thrd_t t1, t2, t3, t4;
  65         lock = new seqlock_t();
  66
  67         thrd_create(&t1, (thrd_start_t)&a, NULL);
  68         thrd_create(&t2, (thrd_start_t)&b, NULL);
  69         thrd_create(&t3, (thrd_start_t)&c, NULL);
  70
  71         thrd_join(t1);
  72         thrd_join(t2);
  73         thrd_join(t3);
  74         return 0;
  75 }