rw spin lock

still only experimental; both the lock itself and its benchmarking

8 files changed, 653 insertions, 48 deletions

diff --git a/vespalib/CMakeLists.txt b/vespalib/CMakeLists.txt
index 6d19988b96b..c1f6f2cbbff 100644
--- a/vespalib/CMakeLists.txt
+++ b/vespalib/CMakeLists.txt
@@ -147,6 +147,7 @@ vespa_define_module(
     src/tests/require
     src/tests/runnable_pair
     src/tests/rusage
+    src/tests/rw_spin_lock
     src/tests/sequencedtaskexecutor
     src/tests/sha1
     src/tests/shared_operation_throttler
diff --git a/vespalib/src/tests/rw_spin_lock/CMakeLists.txt b/vespalib/src/tests/rw_spin_lock/CMakeLists.txt
new file mode 100644
index 00000000000..d05322e79f6
--- /dev/null
+++ b/vespalib/src/tests/rw_spin_lock/CMakeLists.txt
@@ -0,0 +1,8 @@
+# Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+vespa_add_executable(vespalib_rw_spin_lock_test_app TEST
+    SOURCES
+    rw_spin_lock_test.cpp
+    DEPENDS
+    vespalib
+)
+vespa_add_test(NAME vespalib_rw_spin_lock_test_app COMMAND vespalib_rw_spin_lock_test_app)
diff --git a/vespalib/src/tests/rw_spin_lock/rw_spin_lock_test.cpp b/vespalib/src/tests/rw_spin_lock/rw_spin_lock_test.cpp
new file mode 100644
index 00000000000..207284d0db0
--- /dev/null
+++ b/vespalib/src/tests/rw_spin_lock/rw_spin_lock_test.cpp
@@ -0,0 +1,355 @@
+// Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+
+#include <vespa/vespalib/util/spin_lock.h>
+#include <vespa/vespalib/util/rw_spin_lock.h>
+#include <vespa/vespalib/util/atomic.h>
+#include <vespa/vespalib/util/time.h>
+#include <vespa/vespalib/util/classname.h>
+#include <vespa/vespalib/test/thread_meets.h>
+#include <vespa/vespalib/testkit/test_kit.h>
+#include <type_traits>
+#include <ranges>
+#include <random>
+#include <array>
+
+using namespace vespalib;
+using namespace vespalib::atomic;
+
+duration budget = 250ms;
+constexpr size_t LOOP_CNT = 4096;
+constexpr double LOOP_FACTOR = double(LOOP_CNT);
+
+//-----------------------------------------------------------------------------
+
+struct DummyLock {
+    constexpr DummyLock() noexcept {}
+    // BasicLockable
+    constexpr void lock() noexcept {}
+    constexpr void unlock() noexcept {}
+    // SharedLockable
+    constexpr void lock_shared() noexcept {}
+    [[nodiscard]] constexpr bool try_lock_shared() noexcept { return true; }
+    constexpr void unlock_shared() noexcept {}
+    // rw_upgrade_downgrade_lock
+    [[nodiscard]] constexpr bool try_convert_read_to_write() noexcept { return true; }
+    constexpr void convert_write_to_read() noexcept {}
+};
+
+//-----------------------------------------------------------------------------
+
+struct MyState {
+    static constexpr size_t SZ = 5;
+    std::array<size_t,SZ> state = {0,0,0,0,0};
+    std::atomic<size_t> inconsistent_reads = 0;
+    void update() {
+        std::array<size_t,SZ> tmp;
+        for (size_t i = 0; i < SZ; ++i) {
+            tmp[i] = load_ref_relaxed(state[i]);
+        }
+        for (int n = 0; n < 1024; ++n) {
+            for (size_t i = 0; i < SZ; ++i) {
+                store_ref_relaxed(state[i], tmp[i] + 1);
+            }
+        }
+    }
+    void peek() {
+        std::array<size_t,SZ> tmp;
+        for (size_t i = 0; i < SZ; ++i) {
+            tmp[i] = load_ref_relaxed(state[i]);
+        }
+        for (int n = 0; n < 1024; ++n) {
+            for (size_t i = 0; i < SZ; ++i) {
+                if (load_ref_relaxed(state[i]) != tmp[i]) [[unlikely]] {
+                    inconsistent_reads.fetch_add(1, std::memory_order_relaxed);
+                }
+            }
+        }
+    }
+    bool check(size_t expect) const {
+        if (inconsistent_reads > 0) {
+            return false;
+        }
+        for (const auto& value: state) {
+            if (load_ref_relaxed(value) != expect) {
+               return false;
+            }
+        }
+        return true;
+    }
+    void report(size_t expect, const char *name) const {
+        if (check(expect)) {
+            fprintf(stderr, "%s is thread safe\n", name);
+        } else {
+            fprintf(stderr, "%s is not thread safe\n", name);
+            fprintf(stderr, "    inconsistent reads: %zu\n", inconsistent_reads.load());
+            fprintf(stderr, "    expected %zu, got [%zu,%zu,%zu,%zu,%zu]\n",
+                    expect, state[0], state[1], state[2], state[3], state[4]);
+        }
+    }
+};
+
+// do work while waiting for other threads to be ready
+class ActiveBarrier : Rendezvous<bool,bool> {
+private:
+    std::atomic<uint32_t> _ready_cnt;
+    void mingle() override {
+        _ready_cnt.store(0, std::memory_order_relaxed);
+    }
+public:
+    ActiveBarrier(size_t n) : Rendezvous<bool,bool>(n), _ready_cnt(0) {}
+    void operator()(auto &do_work) {
+        if (_ready_cnt.fetch_add(1, std::memory_order_relaxed) + 1 < size()) {
+            do_work();
+        }
+        while (_ready_cnt.load(std::memory_order_relaxed) < size()) {
+            do_work();
+        }
+        rendezvous(false);
+    }
+};
+
+// random generator used to make per-thread decisions
+class Rnd {
+private:
+    std::mt19937 _engine;
+    std::uniform_int_distribution<int> _dist;    
+public:
+    Rnd(uint32_t seed) : _engine(seed), _dist(0,9999) {}
+    bool operator()(int bp) { return _dist(_engine) < bp; }
+};
+
+void fork_join(auto &&thread_fun, size_t num_threads) {
+    assert(num_threads > 0);
+    ThreadPool pool;
+    for (size_t i = 1; i < num_threads; ++i) {
+        pool.start([i,&thread_fun]{ thread_fun(i); });
+    }
+    thread_fun(0);
+    pool.join();
+}
+
+auto apply_merge(auto &&inputs, auto &&perform, auto &&merge) {
+    using output_t = std::decay_t<decltype(perform(*inputs.begin()))>;
+    std::mutex lock;
+    std::optional<output_t> output;
+    auto handle_result = [&](output_t result) {
+                             std::lock_guard guard(lock);
+                             if (output.has_value()) {
+                                 output = merge(std::move(output).value(), std::move(result));
+                             } else {
+                                 output = std::move(result);
+                             }
+                         };
+    ThreadPool pool;
+    for (auto &&item: inputs) {
+        pool.start([item,&perform,&handle_result]{ handle_result(perform(item)); });
+    }
+    pool.join();
+    return output.value();
+}
+
+//-----------------------------------------------------------------------------
+
+template<typename T>
+concept basic_lockable = requires(T a) {
+    { a.lock() } -> std::same_as<void>;
+    { a.unlock() } -> std::same_as<void>;
+};
+
+template<typename T>
+concept lockable = requires(T a) {
+    { a.try_lock() } -> std::same_as<bool>;
+    { a.lock() } -> std::same_as<void>;
+    { a.unlock() } -> std::same_as<void>;
+};
+
+template<typename T>
+concept shared_lockable = requires(T a) {
+    { a.try_lock_shared() } -> std::same_as<bool>;
+    { a.lock_shared() } -> std::same_as<void>;
+    { a.unlock_shared() } -> std::same_as<void>;
+};
+
+template<typename T>
+concept can_upgrade = requires(std::shared_lock<T> a, std::unique_lock<T> b) {
+    { try_upgrade(std::move(a)) } -> std::same_as<std::unique_lock<T>>;
+    { downgrade(std::move(b)) } -> std::same_as<std::shared_lock<T>>;
+};
+
+//-----------------------------------------------------------------------------
+
+template <size_t N>
+auto run_loop(auto &f) {
+    static_assert(N % 4 == 0);
+    for (size_t i = 0; i < N / 4; ++i) {
+        f(); f(); f(); f();
+    }
+}
+
+double measure_ns(auto &work) __attribute__((noinline));
+double measure_ns(auto &work) {
+    auto t0 = steady_clock::now();
+    run_loop<LOOP_CNT>(work);
+    return count_ns(steady_clock::now() - t0) / LOOP_FACTOR;
+}
+
+struct BenchmarkResult {
+    double cost_ns = std::numeric_limits<double>::max();
+    double range_ns = 0.0;
+};
+
+struct Meets {
+    vespalib::test::ThreadMeets::Vote vote;
+    vespalib::test::ThreadMeets::Avg avg;
+    vespalib::test::ThreadMeets::Range<double> range;
+    ActiveBarrier active_wait;
+    Meets(size_t num_threads)
+      : vote(num_threads), avg(num_threads), range(num_threads), active_wait(num_threads) {}
+    ~Meets();
+};
+Meets::~Meets() = default;
+
+BenchmarkResult benchmark_ns(auto &&work, size_t num_threads = 1) {
+    Timer timer;
+    Meets meets(num_threads);
+    BenchmarkResult result;
+    auto hook = [&](size_t thread_id) {
+        for (bool once_more = true; meets.vote(once_more); once_more = (timer.elapsed() < budget)) {
+            auto my_ns = measure_ns(work);
+            meets.active_wait(work);
+            auto cost_ns = meets.avg(my_ns);
+            auto range_ns = meets.range(my_ns);
+            if (thread_id == 0 && cost_ns < result.cost_ns) {
+                result.cost_ns = cost_ns;
+                result.range_ns = range_ns;
+            }
+        }
+    };
+    fork_join(hook, num_threads);
+    return result;
+}
+
+//-----------------------------------------------------------------------------
+
+template <typename T>
+void estimate_cost() {
+    T lock;
+    auto name = getClassName(lock);
+    static_assert(basic_lockable<T>);
+    fprintf(stderr, "%s unique lock/unlock: %g ns\n", name.c_str(),
+            benchmark_ns([&lock]{ lock.lock(); lock.unlock(); }).cost_ns);
+    if constexpr (shared_lockable<T>) {
+        fprintf(stderr, "%s shared lock/unlock: %g ns\n", name.c_str(),
+                benchmark_ns([&lock]{ lock.lock_shared(); lock.unlock_shared(); }).cost_ns);
+    }
+    if constexpr (can_upgrade<T>) {
+        auto guard = std::shared_lock(lock);
+        fprintf(stderr, "%s upgrade/downgrade: %g ns\n", name.c_str(),
+                benchmark_ns([&lock]{
+                                 assert(lock.try_convert_read_to_write());
+                                 lock.convert_write_to_read();
+                             }).cost_ns);
+    }
+}
+
+//-----------------------------------------------------------------------------
+
+template <typename T>
+size_t thread_safety_loop(T &lock, MyState &state, Meets &meets, int read_bp, size_t thread_id) {
+    Timer timer;
+    Rnd rnd(thread_id);
+    size_t write_cnt = 0;
+    BenchmarkResult result;
+    auto do_work = [&]
+                   {
+                       if (rnd(read_bp)) {
+                           if constexpr (shared_lockable<T>) {
+                               std::shared_lock guard(lock);
+                               state.peek();
+                           } else {
+                               std::lock_guard guard(lock);
+                               state.peek();
+                           }
+                       } else {
+                           {
+                               std::lock_guard guard(lock);
+                               state.update();                
+                           }
+                           ++write_cnt;
+                       }
+                   };
+    for (bool once_more = true; meets.vote(once_more); once_more = (timer.elapsed() < budget)) {
+        auto my_est = measure_ns(do_work);
+        meets.active_wait(do_work);
+        auto cost_ns = meets.avg(my_est);
+        auto range_ns = meets.range(my_est);
+        if (cost_ns < result.cost_ns) {
+            result.cost_ns = cost_ns;
+            result.range_ns = range_ns;
+        }
+    }
+    if (thread_id == 0) {
+        fprintf(stderr, "---> %s with %2zu threads (%5d bp r): %12.2f ns, range: %12.2f ns\n",
+                getClassName(lock).c_str(), meets.vote.size(), read_bp, result.cost_ns, result.range_ns);
+    }
+    return write_cnt;
+}
+
+//-----------------------------------------------------------------------------
+
+TEST("require that rw spin locks can be used with lock_guard, unique_lock and shared_lock") {
+    static_assert(basic_lockable<RWSpinLock>);
+    static_assert(lockable<RWSpinLock>);
+    static_assert(shared_lockable<RWSpinLock>);
+    static_assert(can_upgrade<RWSpinLock>);
+    RWSpinLock lock;
+    { auto guard = std::lock_guard(lock); }
+    { auto guard = std::unique_lock(lock); }
+    { auto guard = std::shared_lock(lock); }
+}
+
+TEST("estimate basic cost") {
+    Rnd rnd(123);
+    MyState state;
+    fprintf(stderr, "   rnd cost: %8.2f ns\n", benchmark_ns([&]{ rnd(50); }).cost_ns);
+    fprintf(stderr, "  peek cost: %8.2f ns\n", benchmark_ns([&]{ state.peek(); }).cost_ns);
+    fprintf(stderr, "update cost: %8.2f ns\n", benchmark_ns([&]{ state.update(); }).cost_ns);
+}
+
+void benchmark_lock(auto &lock) {
+    size_t expect = 0;
+    auto state = std::make_unique<MyState>();
+    for (size_t bp: {10000, 9999, 5000, 0}) {
+        for (size_t num_threads: {8, 4, 2, 1}) {
+            Meets meets(num_threads);
+            auto hook = [&](size_t thread_id) {
+                            return thread_safety_loop(lock, *state, meets, bp, thread_id);
+                        };
+            expect += apply_merge(std::views::iota(size_t(0), num_threads), hook, [](auto a, auto b){ return a + b; });
+        }
+    }
+    state->report(expect, getClassName(lock).c_str());
+    EXPECT_TRUE(state->check(expect));
+}
+
+TEST_F("benchmark RWSpinLock", RWSpinLock()) { benchmark_lock(f1); }
+TEST_F("benchmark std::shared_mutex", std::shared_mutex()) { benchmark_lock(f1); }
+TEST_F("benchmark std::mutex", std::mutex()) { benchmark_lock(f1); }
+TEST_F("benchmark SpinLock", SpinLock()) { benchmark_lock(f1); }
+
+TEST("estimate single-threaded lock/unlock cost") {
+    estimate_cost<DummyLock>();
+    estimate_cost<SpinLock>();
+    estimate_cost<std::mutex>();
+    estimate_cost<RWSpinLock>();
+    estimate_cost<std::shared_mutex>();
+}
+
+int main(int argc, char **argv) {
+    TEST_MASTER.init(__FILE__);
+    if ((argc == 2) && (argv[1] == std::string("bench"))) {
+        budget = 5s;
+    }
+    TEST_RUN_ALL();
+    return (TEST_MASTER.fini() ? 0 : 1);
+}
diff --git a/vespalib/src/tests/shared_string_repo/shared_string_repo_test.cpp b/vespalib/src/tests/shared_string_repo/shared_string_repo_test.cpp
index dfcba14ba63..910c2d017ba 100644
--- a/vespalib/src/tests/shared_string_repo/shared_string_repo_test.cpp
+++ b/vespalib/src/tests/shared_string_repo/shared_string_repo_test.cpp
@@ -1,7 +1,7 @@
 // Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
 
 #include <vespa/vespalib/util/shared_string_repo.h>
-#include <vespa/vespalib/util/rendezvous.h>
+#include <vespa/vespalib/test/thread_meets.h>
 #include <vespa/vespalib/util/time.h>
 #include <vespa/vespalib/util/size_literals.h>
 #include <vespa/vespalib/util/stringfmt.h>
@@ -115,41 +115,8 @@ std::unique_ptr<StringIdVector> make_weak_handles(const Handles &handles) {
 
 //-----------------------------------------------------------------------------
 
-struct Avg : Rendezvous<double, double> {
-    explicit Avg(size_t n) : Rendezvous<double, double>(n) {}
-    void mingle() override {
-        double sum = 0;
-        for (size_t i = 0; i < size(); ++i) {
-            sum += in(i);
-        }
-        double result = sum / size();
-        for (size_t i = 0; i < size(); ++i) {
-            out(i) = result;
-        }
-    }
-    double operator()(double value) { return rendezvous(value); }
-};
-
-struct Vote : Rendezvous<bool, bool> {
-    explicit Vote(size_t n) : Rendezvous<bool, bool>(n) {}
-    void mingle() override {
-        size_t true_cnt = 0;
-        size_t false_cnt = 0;
-        for (size_t i = 0; i < size(); ++i) {
-            if (in(i)) {
-                ++true_cnt;
-            } else {
-                ++false_cnt;
-            }
-        }
-        bool result = (true_cnt > false_cnt);
-        for (size_t i = 0; i < size(); ++i) {
-            out(i) = result;
-        }
-    }
-    [[nodiscard]] size_t num_threads() const { return size(); }
-    bool operator()(bool flag) { return rendezvous(flag); }
-};
+using Avg = vespalib::test::ThreadMeets::Avg;
+using Vote = vespalib::test::ThreadMeets::Vote;
 
 //-----------------------------------------------------------------------------
 
@@ -174,7 +141,7 @@ struct Fixture {
         : avg(num_threads), vote(num_threads), work(make_strings(work_size)), direct_work(make_direct_strings(work_size)), start_time(steady_clock::now()) {}
     ~Fixture() {
         if (verbose) {
-            fprintf(stderr, "benchmark results for %zu threads:\n", vote.num_threads());
+            fprintf(stderr, "benchmark results for %zu threads:\n", vote.size());
             for (const auto &[tag, ms_cost]: time_ms) {
                 fprintf(stderr, "    %s: %g ms\n", tag.c_str(), ms_cost);
             }
diff --git a/vespalib/src/vespa/vespalib/test/thread_meets.cpp b/vespalib/src/vespa/vespalib/test/thread_meets.cpp
index 9d23e0eab28..607179c53f9 100644
--- a/vespalib/src/vespa/vespalib/test/thread_meets.cpp
+++ b/vespalib/src/vespa/vespalib/test/thread_meets.cpp
@@ -9,4 +9,35 @@ ThreadMeets::Nop::mingle()
 {
 }
 
+void
+ThreadMeets::Avg::mingle()
+{
+    double sum = 0;
+    for (size_t i = 0; i < size(); ++i) {
+        sum += in(i);
+    }
+    double result = sum / size();
+    for (size_t i = 0; i < size(); ++i) {
+        out(i) = result;
+    }
+}
+
+void
+ThreadMeets::Vote::mingle()
+{
+    size_t true_cnt = 0;
+    size_t false_cnt = 0;
+    for (size_t i = 0; i < size(); ++i) {
+        if (in(i)) {
+            ++true_cnt;
+        } else {
+            ++false_cnt;
+        }
+    }
+    bool result = (true_cnt > false_cnt);
+    for (size_t i = 0; i < size(); ++i) {
+        out(i) = result;
+    }
+}
+
 }
diff --git a/vespalib/src/vespa/vespalib/test/thread_meets.h b/vespalib/src/vespa/vespalib/test/thread_meets.h
index 62ca7779935..7ef4dcb9921 100644
--- a/vespalib/src/vespa/vespalib/test/thread_meets.h
+++ b/vespalib/src/vespa/vespalib/test/thread_meets.h
@@ -12,10 +12,67 @@ namespace vespalib::test {
 struct ThreadMeets {
     // can be used as a simple thread barrier
     struct Nop : vespalib::Rendezvous<bool,bool> {
-        Nop(size_t N) : vespalib::Rendezvous<bool,bool>(N) {}
+        explicit Nop(size_t N) : vespalib::Rendezvous<bool,bool>(N) {}
         void operator()() { rendezvous(false); }
         void mingle() override;
     };
+    // calculate the average value across threads
+    struct Avg : Rendezvous<double, double> {
+        explicit Avg(size_t n) : Rendezvous<double, double>(n) {}
+        double operator()(double value) { return rendezvous(value); }
+        void mingle() override;
+    };
+    // threads vote for true/false, majority wins (false on tie)
+    struct Vote : Rendezvous<bool, bool> {
+        explicit Vote(size_t n) : Rendezvous<bool, bool>(n) {}
+        bool operator()(bool flag) { return rendezvous(flag); }
+        void mingle() override;
+    };
+    // sum of values across all threads
+    template <typename T>
+    struct Sum : vespalib::Rendezvous<T,T> {
+        using vespalib::Rendezvous<T,T>::in;
+        using vespalib::Rendezvous<T,T>::out;
+        using vespalib::Rendezvous<T,T>::size;
+        using vespalib::Rendezvous<T,T>::rendezvous;
+        explicit Sum(size_t N) : vespalib::Rendezvous<T,T>(N) {}
+        T operator()(T value) { return rendezvous(value); }
+        void mingle() override {
+            T acc{};
+            for (size_t i = 0; i < size(); ++i) {
+                acc += in(i);
+            }
+            for (size_t i = 0; i < size(); ++i) {
+                out(i) = acc;
+            }
+        }
+    };
+    // range of values across all threads
+    template <typename T>
+    struct Range : vespalib::Rendezvous<T,T> {
+        using vespalib::Rendezvous<T,T>::in;
+        using vespalib::Rendezvous<T,T>::out;
+        using vespalib::Rendezvous<T,T>::size;
+        using vespalib::Rendezvous<T,T>::rendezvous;
+        explicit Range(size_t N) : vespalib::Rendezvous<T,T>(N) {}
+        T operator()(T value) { return rendezvous(value); }
+        void mingle() override {
+            T min = in(0);
+            T max = in(0);
+            for (size_t i = 1; i < size(); ++i) {
+                if (in(i) < min) {
+                    min = in(i);
+                }
+                if (in(i) > max) {
+                    max = in(i);
+                }
+            }
+            T result = (max - min);
+            for (size_t i = 0; i < size(); ++i) {
+                out(i) = result;
+            }
+        }
+    };
     // swap values between 2 threads
     template <typename T>
     struct Swap : vespalib::Rendezvous<T,T> {
@@ -25,8 +82,8 @@ struct ThreadMeets {
         Swap() : vespalib::Rendezvous<T,T>(2) {}
         T operator()(T input) { return rendezvous(input); }
         void mingle() override {
-            out(1) = in(0);
-            out(0) = in(1);
+            out(1) = std::move(in(0));
+            out(0) = std::move(in(1));
         }
     };
 };
diff --git a/vespalib/src/vespa/vespalib/util/rendezvous.h b/vespalib/src/vespa/vespalib/util/rendezvous.h
index 2880f325d96..17a8729c54c 100644
--- a/vespalib/src/vespa/vespalib/util/rendezvous.h
+++ b/vespalib/src/vespa/vespalib/util/rendezvous.h
@@ -50,14 +50,6 @@ private:
 
 protected:
     /**
-     * Obtain the number of input and output values to be handled by
-     * mingle. This function is called by mingle.
-     *
-     * @return number of input and output values
-     **/
-    size_t size() const { return _size; }
-
-    /**
      * Obtain an input parameter. This function is called by mingle.
      *
      * @return reference to the appropriate input
@@ -87,6 +79,11 @@ public:
     virtual ~Rendezvous();
 
     /**
+     * @return number of participants
+     **/
+    size_t size() const { return _size; }
+
+    /**
      * Called by individual threads to synchronize execution and share
      * state with the mingle function.
      *
diff --git a/vespalib/src/vespa/vespalib/util/rw_spin_lock.h b/vespalib/src/vespa/vespalib/util/rw_spin_lock.h
new file mode 100644
index 00000000000..f2c15dcc0eb
--- /dev/null
+++ b/vespalib/src/vespa/vespalib/util/rw_spin_lock.h
@@ -0,0 +1,189 @@
+// Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+
+#pragma once
+
+#include <mutex>
+#include <shared_mutex>
+#include <atomic>
+#include <thread>
+#include <cassert>
+#include <utility>
+
+namespace vespalib {
+
+/**
+ * A reader-writer spin lock implementation.
+ *
+ * reader:  shared access for any number of readers
+ * writer:  exclusive access for a single writer
+ *
+ * valid lock combinations:
+ *   {}
+ *   {N readers}
+ *   {1 writer}
+ *
+ * Trying to obtain a write lock will lead to not granting new read
+ * locks.
+ *
+ * This lock is intended for use-cases that involves mostly reading,
+ * with a little bit of writing.
+ *
+ * This class implements the Lockable and SharedLockable named
+ * requirements from the standard library, making it directly usable
+ * with std::shared_lock (reader) and std::unique_lock (writer)
+ *
+ * There is also some special glue added for lock upgrading and
+ * downgrading.
+ *
+ * NOTE: this implementation is experimental, mostly intended for
+ *       benchmarking and trying to identify use-cases that work with
+ *       rw locks. Upgrade locks that do not block readers might be
+ *       implementet in the future.
+ **/
+class RWSpinLock {
+private:
+    // [31: num readers][1: pending writer]
+    // a reader gets the lock by:
+    //   increasing the number of readers while the pending writer bit is not set.
+    // a writer gets the lock by:
+    //   changing the pending writer bit from 0 to 1 and then
+    //   waiting for the number of readers to become 0
+    // an upgrade is successful when:
+    //   a reader is able to obtain the pending writer bit
+    std::atomic<uint32_t> _state;
+
+    // Convenience function used to check if the pending writer bit is
+    // set in the given value.
+    bool has_pending_writer(uint32_t value) noexcept {
+        return (value & 1);
+    }
+
+    // Wait for all readers to release their locks.
+    void wait_for_zero_readers(uint32_t &value) {
+        while (value != 1) {
+            std::this_thread::yield();
+            value = _state.load(std::memory_order_acquire);
+        }
+    }
+
+public:
+    RWSpinLock() noexcept : _state(0) {
+        static_assert(std::atomic<uint32_t>::is_always_lock_free);
+    }
+
+    // implementation of Lockable named requirement - vvv
+
+    void lock() noexcept {
+        uint32_t expected = 0;
+        uint32_t desired = 1;
+        while (!_state.compare_exchange_weak(expected, desired,
+                                             std::memory_order_acquire,
+                                             std::memory_order_relaxed))
+        {
+            while (has_pending_writer(expected)) {
+                std::this_thread::yield();
+                expected = _state.load(std::memory_order_relaxed);
+            }
+            desired = expected + 1;
+        }
+        wait_for_zero_readers(desired);
+    }
+
+    [[nodiscard]] bool try_lock() noexcept {
+        uint32_t expected = 0;
+        return _state.compare_exchange_strong(expected, 1,
+                                              std::memory_order_acquire,
+                                              std::memory_order_relaxed);
+    }
+
+    void unlock() noexcept {
+        _state.store(0, std::memory_order_release);
+    }
+
+    // implementation of Lockable named requirement - ^^^
+
+    // implementation of SharedLockable named requirement - vvv
+
+    void lock_shared() noexcept {
+        uint32_t expected = 0;
+        uint32_t desired = 2;
+        while (!_state.compare_exchange_weak(expected, desired,
+                                             std::memory_order_acquire,
+                                             std::memory_order_relaxed))
+        {
+            while (has_pending_writer(expected)) {
+                std::this_thread::yield();
+                expected = _state.load(std::memory_order_relaxed);
+            }
+            desired = expected + 2;
+        }
+    }
+
+    [[nodiscard]] bool try_lock_shared() noexcept {
+        uint32_t expected = 0;
+        uint32_t desired = 2;
+        while (!_state.compare_exchange_weak(expected, desired,
+                                             std::memory_order_acquire,
+                                             std::memory_order_relaxed))
+        {
+            if (has_pending_writer(expected)) {
+                return false;
+            }
+            desired = expected + 2;
+        }
+        return true;
+    }
+
+    void unlock_shared() noexcept {
+        _state.fetch_sub(2, std::memory_order_release);
+    }
+
+    // implementation of SharedLockable named requirement - ^^^
+
+    // try to upgrade a read (shared) lock to a write (unique) lock
+    bool try_convert_read_to_write() noexcept {
+        uint32_t expected = 2;
+        uint32_t desired = 1;
+        while (!_state.compare_exchange_weak(expected, desired,
+                                             std::memory_order_acquire,
+                                             std::memory_order_relaxed))
+        {
+            if (has_pending_writer(expected)) {
+                return false;
+            }
+            desired = expected - 1;
+        }
+        wait_for_zero_readers(desired);
+        return true;
+    }
+
+    // convert a write (unique) lock to a read (shared) lock
+    void convert_write_to_read() noexcept {
+        _state.store(2, std::memory_order_release);        
+    }
+};
+
+template<typename T>
+concept rw_upgrade_downgrade_lock = requires(T a, T b) {
+    { a.try_convert_read_to_write() } -> std::same_as<bool>;
+    { b.convert_write_to_read() } -> std::same_as<void>;
+};
+
+template <rw_upgrade_downgrade_lock T>
+[[nodiscard]] std::unique_lock<T> try_upgrade(std::shared_lock<T> &&guard) noexcept {
+    assert(guard.owns_lock());
+    if (guard.mutex()->try_convert_read_to_write()) {
+        return {*guard.release(), std::adopt_lock};
+    } else {
+        return {};
+    }
+}
+
+template <rw_upgrade_downgrade_lock T>
+[[nodiscard]] std::shared_lock<T> downgrade(std::unique_lock<T> &&guard) noexcept {
+    assert(guard.owns_lock());
+    guard.mutex()->convert_write_to_read();
+    return {*guard.release(), std::adopt_lock};
+}
+
+}