Build fails, missing ReaderWriterLock2

[m-mueller678]

Build fails, missing ReaderWriterLock2

[m-mueller678]

• The ordering of cd and git checkout in the doc is flipped
• the doc refers to btree_tests/ParallelTools/ParallelTools/Lock.cpp, which does not exist (it is header only).
• ReaderWriterLock2 is not in the Lock header

manually adding ReaderWriterLock2 it compiles and seems to work. Thank you for your help.

[wheatman]

Thanks for the report. We will try and improve the readme and fix it so it just compiles out of the box.

As for ReaderwriterLock2 the definition is bellow. Let me know if that works and we will update the repo with it as well. You can also just change ReaderWriterLock2 to ReaderWriterLock. They have some different space usage and performance characteristics in the contested and uncontested cases and I would recommend using ReaderWriterLock2 in most cases.

class ReaderWriterLock2 {

public:
  ReaderWriterLock2() : writer(0), readers(0) {}

  /**
   * Try to acquire a lock and spin until the lock is available.
   */
  void read_lock(int cpuid = -1) {

    readers++;

    while (writer.test(std::memory_order_relaxed)) {
      readers--;
      writer.wait(true, std::memory_order_relaxed);
      readers++;
    }
  }

  void read_unlock(int cpuid) {
    readers--;
    return;
  }

  /**
   * Try to acquire a write lock and spin until the lock is available.
   * Then wait till reader count is 0.
   */
  void write_lock() {
    // acquire write lock.
    while (writer.test_and_set(std::memory_order_acq_rel)) {
      writer.wait(true, std::memory_order_acq_rel);
    }
    // wait for readers to finish
    while (readers > 0) {
    }
  }

  void write_unlock(void) {
    writer.clear(std::memory_order_release);
    writer.notify_all();
    return;
  }

private:
  std::atomic_flag writer{false};
  std::atomic<int> readers{};
};

[m-mueller678]

After adding the definition to btree.hpp, I get this (inside function exists):

../tlx/container/btree.hpp:1748:38: error: too few arguments to function call, single argument 'cpuid' was not specified
            leaf->mutex_.read_unlock();

replacing it with ReaderWriterLock yields the same error

[wheatman]

my mistake

public:
  ReaderWriterLock2() : writer(0), readers(0) {}

  /**
   * Try to acquire a lock and spin until the lock is available.
   */
  void read_lock(int cpuid = -1) {

    readers++;

    while (writer.test(std::memory_order_relaxed)) {
      readers--;
      writer.wait(true, std::memory_order_relaxed);
      readers++;
    }
  }

  void read_unlock(int cpuid = -1) {
    readers--;
    return;
  }

  /**
   * Try to acquire a write lock and spin until the lock is available.
   * Then wait till reader count is 0.
   */
  void write_lock() {
    // acquire write lock.
    while (writer.test_and_set(std::memory_order_acq_rel)) {
      writer.wait(true, std::memory_order_acq_rel);
    }
    // wait for readers to finish
    while (readers > 0) {
    }
  }

  void write_unlock(void) {
    writer.clear(std::memory_order_release);
    writer.notify_all();
    return;
  }

private:
  std::atomic_flag writer{false};
  std::atomic<int> readers{};
};

[itshelenxu]

Please check the new main branch, which I cleaned up to remove the dependency on cilk and to make running the experiments smoother. ParallelTools has been updated with ReaderWriterLock2. Here is more documentation:
https://docs.google.com/document/d/1GqQBpYTzSixPAQMfuK8pHyDVL7OjqDHb-Q0u406x894/edit#heading=h.dcbs04rhzkfs

Let us know if there are any questions / issues. Thank you!

[itshelenxu]

Thank you for the notes - I have updated the doc to fix the first two issues.

We have updated the ParallelTools repo to include ReaderWriterLock2: https://github.com/wheatman/ParallelTools/blob/main/ParallelTools/Lock.hpp

Please update the submodule to get it.