cds/sync/seqlock.h

   1 #ifndef _SEQLOCK_H
   2 #define _SEQLOCK_H
   3
   4 #include <atomic>
   5
   6 namespace cds_others {
   7
   8 using std::atomic_int;
   9 using std::memory_order_release;
  10 using std::memory_order_acquire;
  11 using std::memory_order_relaxed;
  12
  13 class SeqLock {
  14 private:
  15   // Sequence for reader consistency check.
  16   atomic_int seq_;
  17   // It needs to be atomic to avoid data races
  18   atomic_int data_;
  19
  20 public:
  21   SeqLock() {
  22     atomic_init(&seq_, 0);
  23     atomic_init(&data_, 0);
  24   }
  25
  26   int read() {
  27     while (true) {
  28       int old_seq = seq_.load(memory_order_acquire); // acquire
  29       if (old_seq % 2 == 1)
  30         continue;
  31
  32       int res = data_.load(memory_order_acquire);       // acquire
  33       if (seq_.load(memory_order_relaxed) == old_seq) { // relaxed
  34         return res;
  35       }
  36     }
  37   }
  38
  39   void write(int new_data) {
  40     while (true) {
  41       // This might be a relaxed too
  42       int old_seq = seq_.load(memory_order_acquire); // acquire
  43       if (old_seq % 2 == 1)
  44         continue; // Retry
  45
  46       // Should be relaxed!!!
  47       if (seq_.compare_exchange_strong(old_seq, old_seq + 1,
  48                                        memory_order_relaxed,
  49                                        memory_order_relaxed)) // relaxed
  50         break;
  51     }
  52
  53     // Update the data
  54     data_.store(new_data, memory_order_release); // release
  55
  56     seq_.fetch_add(1, memory_order_release); // release
  57   }
  58 };
  59
  60 } // namespace cds_others
  61
  62 #endif