cds/misc/seqlock.h

   1 #ifndef _SEQLOCK_H
   2 #define _SEQLOCK_H
   3
   4 #include "backoff.h"
   5 #include <atomic>
   6
   7 namespace cds_others {
   8
   9 using std::atomic_int;
  10 using std::memory_order_release;
  11 using std::memory_order_acquire;
  12 using std::memory_order_relaxed;
  13
  14 class SeqLock {
  15 private:
  16   // Sequence for reader consistency check.
  17   atomic_int seq_;
  18   // It needs to be atomic to avoid data races
  19   atomic_int data_;
  20
  21 public:
  22   SeqLock() {
  23     atomic_init(&seq_, 0);
  24     atomic_init(&data_, 0);
  25   }
  26
  27   int read() {
  28     ExpBackoff backoff;
  29     while (true) {
  30       int old_seq = seq_.load(memory_order_acquire);
  31       if (old_seq % 2 == 1) {
  32         backoff();
  33         continue;
  34       }
  35
  36       int res = data_.load(memory_order_acquire);
  37       if (seq_.load(memory_order_relaxed) == old_seq) {
  38         return res;
  39       }
  40     }
  41   }
  42
  43   void write(int new_data) {
  44     ExpBackoff backoff;
  45     while (true) {
  46       // This might be a relaxed too
  47       int old_seq = seq_.load(memory_order_acquire);
  48       if (old_seq % 2 == 1) {
  49         backoff();
  50         continue; // Retry
  51       }
  52
  53       // Should be relaxed!!!
  54       if (seq_.compare_exchange_strong(old_seq, old_seq + 1,
  55                                        memory_order_relaxed,
  56                                        memory_order_relaxed)) {
  57         break;
  58       }
  59     }
  60
  61     // Update the data
  62     data_.store(new_data, memory_order_release); // release
  63
  64     seq_.fetch_add(1, memory_order_release); // release
  65   }
  66 };
  67
  68 } // namespace cds_others
  69
  70 #endif