swmr_spin_lock.cpp

#include "pls/internal/base/swmr_spin_lock.h"
#include "pls/internal/base/system_details.h"

namespace pls {
namespace internal {
namespace base {

bool swmr_spin_lock::reader_try_lock() {
  PROFILE_LOCK("Try Acquire Read Lock")
  if (write_request_.load(std::memory_order_relaxed) == 1) {
    return false;
  }
  // We think we can enter the region
  readers_.fetch_add(1, std::memory_order_acquire);
  if (write_request_.load() == 1) {
    // Whoops, the writer acquires the lock, so we back off again
    readers_--;
    return false;
  }

  return true;
}

void swmr_spin_lock::reader_unlock() {
  PROFILE_LOCK("Release Read Lock")
  readers_.fetch_add(-1, std::memory_order_release);
}

void swmr_spin_lock::writer_lock() {
  PROFILE_LOCK("Acquire Write Lock")
  // Tell the readers that we would like to write
  int expected;
  while (true) {
    expected = 0;
    if (write_request_.compare_exchange_weak(expected, 1, std::memory_order_acquire)) {
      break;
    }
    system_details::relax_cpu(); // Spin until WE set the write lock flag
  }

  // Wait for all of them to exit the critical section
  while (readers_.load() > 0)
    system_details::relax_cpu(); // Spin, not expensive as relaxed load
}

void swmr_spin_lock::writer_unlock() {
  PROFILE_LOCK("Release Write Lock")
  write_request_.store(0, std::memory_order_release);
}

}
}
}