1 files changed, 156 insertions, 0 deletions
diff --git a/src/crimson/common/tri_mutex.h b/src/crimson/common/tri_mutex.h
new file mode 100644
index 000000000..127573b3a
--- /dev/null
+++ b/src/crimson/common/tri_mutex.h
@@ -0,0 +1,156 @@
+// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:nil -*-
+// vim: ts=8 sw=2 smarttab
+
+#pragma once
+
+#include <seastar/core/future.hh>
+#include <seastar/core/circular_buffer.hh>
+
+class read_lock {
+public:
+  seastar::future<> lock();
+  void unlock();
+};
+
+class write_lock {
+public:
+  seastar::future<> lock();
+  void unlock();
+};
+
+class excl_lock {
+public:
+  seastar::future<> lock();
+  void unlock();
+};
+
+// promote from read to excl
+class excl_lock_from_read {
+public:
+  seastar::future<> lock();
+  void unlock();
+};
+
+// promote from write to excl
+class excl_lock_from_write {
+public:
+  seastar::future<> lock();
+  void unlock();
+};
+
+// promote from excl to excl
+class excl_lock_from_excl {
+public:
+  seastar::future<> lock();
+  void unlock();
+};
+
+/// shared/exclusive mutual exclusion
+///
+/// this lock design uses reader and writer is entirely and completely
+/// independent of the conventional reader/writer lock usage. Here, what we
+/// mean is that we can pipeline reads, and we can pipeline writes, but we
+/// cannot allow a read while writes are in progress or a write while reads are
+/// in progress. Any rmw operation is therefore exclusive.
+///
+/// tri_mutex is based on seastar::shared_mutex, but instead of two kinds of
+/// waiters, tri_mutex keeps track of three kinds of lock users:
+/// - readers
+/// - writers
+/// - exclusive users
+class tri_mutex : private read_lock,
+                          write_lock,
+                          excl_lock,
+                          excl_lock_from_read,
+                          excl_lock_from_write,
+                          excl_lock_from_excl
+{
+public:
+  tri_mutex() = default;
+  ~tri_mutex();
+
+  read_lock& for_read() {
+    return *this;
+  }
+  write_lock& for_write() {
+    return *this;
+  }
+  excl_lock& for_excl() {
+    return *this;
+  }
+  excl_lock_from_read& excl_from_read() {
+    return *this;
+  }
+  excl_lock_from_write& excl_from_write() {
+    return *this;
+  }
+  excl_lock_from_write& excl_from_excl() {
+    return *this;
+  }
+
+  // for shared readers
+  seastar::future<> lock_for_read();
+  bool try_lock_for_read() noexcept;
+  void unlock_for_read();
+  void promote_from_read();
+  void demote_to_read();
+  unsigned get_readers() const {
+    return readers;
+  }
+
+  // for shared writers
+  seastar::future<> lock_for_write(bool greedy);
+  bool try_lock_for_write(bool greedy) noexcept;
+  void unlock_for_write();
+  void promote_from_write();
+  void demote_to_write();
+  unsigned get_writers() const {
+    return writers;
+  }
+
+  // for exclusive users
+  seastar::future<> lock_for_excl();
+  bool try_lock_for_excl() noexcept;
+  void unlock_for_excl();
+  bool is_excl_acquired() const {
+    return exclusively_used;
+  }
+
+  bool is_acquired() const;
+
+  /// pass the provided exception to any waiting waiters
+  template<typename Exception>
+  void abort(Exception ex) {
+    while (!waiters.empty()) {
+      auto& waiter = waiters.front();
+      waiter.pr.set_exception(std::make_exception_ptr(ex));
+      waiters.pop_front();
+    }
+  }
+
+private:
+  void wake();
+  unsigned readers = 0;
+  unsigned writers = 0;
+  bool exclusively_used = false;
+  enum class type_t : uint8_t {
+    read,
+    write,
+    exclusive,
+    none,
+  };
+  struct waiter_t {
+    waiter_t(seastar::promise<>&& pr, type_t type)
+      : pr(std::move(pr)), type(type)
+    {}
+    seastar::promise<> pr;
+    type_t type;
+  };
+  seastar::circular_buffer<waiter_t> waiters;
+  friend class read_lock;
+  friend class write_lock;
+  friend class excl_lock;
+  friend class excl_lock_from_read;
+  friend class excl_lock_from_write;
+  friend class excl_lock_from_excl;
+};