1 files changed, 112 insertions, 0 deletions
diff --git a/storage/innobase/include/rw_lock.h b/storage/innobase/include/rw_lock.h
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+++ b/storage/innobase/include/rw_lock.h
@@ -0,0 +1,112 @@
+/*****************************************************************************
+
+Copyright (c) 2020, MariaDB Corporation.
+
+This program is free software; you can redistribute it and/or modify it under
+the terms of the GNU General Public License as published by the Free Software
+Foundation; version 2 of the License.
+
+This program is distributed in the hope that it will be useful, but WITHOUT
+ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License along with
+this program; if not, write to the Free Software Foundation, Inc.,
+51 Franklin Street, Fifth Floor, Boston, MA 02110-1335 USA
+
+*****************************************************************************/
+
+#pragma once
+#include <atomic>
+#include "my_dbug.h"
+
+/** Simple read-write lock based on std::atomic */
+class rw_lock
+{
+  /** The lock word */
+  std::atomic<uint32_t> lock;
+
+protected:
+  /** Available lock */
+  static constexpr uint32_t UNLOCKED= 0;
+  /** Flag to indicate that write_lock() is being held */
+  static constexpr uint32_t WRITER= 1U << 31;
+  /** Flag to indicate that write_lock_wait() is pending */
+  static constexpr uint32_t WRITER_WAITING= 1U << 30;
+  /** Flag to indicate that write_lock() or write_lock_wait() is pending */
+  static constexpr uint32_t WRITER_PENDING= WRITER | WRITER_WAITING;
+
+  /** Start waiting for an exclusive lock. */
+  void write_lock_wait_start()
+  { lock.fetch_or(WRITER_WAITING, std::memory_order_relaxed); }
+  /** Try to acquire a shared lock.
+  @param l the value of the lock word
+  @return whether the lock was acquired */
+  bool read_trylock(uint32_t &l)
+  {
+    l= UNLOCKED;
+    while (!lock.compare_exchange_strong(l, l + 1, std::memory_order_acquire,
+                                         std::memory_order_relaxed))
+    {
+      DBUG_ASSERT(!(WRITER & l) || !(~WRITER_PENDING & l));
+      if (l & WRITER_PENDING)
+        return false;
+    }
+    return true;
+  }
+  /** Wait for an exclusive lock.
+  @return whether the exclusive lock was acquired */
+  bool write_lock_poll()
+  {
+    auto l= WRITER_WAITING;
+    if (lock.compare_exchange_strong(l, WRITER, std::memory_order_acquire,
+                                     std::memory_order_relaxed))
+      return true;
+    if (!(l & WRITER_WAITING))
+      /* write_lock() must have succeeded for another thread */
+      write_lock_wait_start();
+    return false;
+  }
+
+public:
+  /** Default constructor */
+  rw_lock() : lock(UNLOCKED) {}
+
+  /** Release a shared lock */
+  void read_unlock()
+  {
+    IF_DBUG_ASSERT(auto l=,) lock.fetch_sub(1, std::memory_order_release);
+    DBUG_ASSERT(l & ~WRITER_PENDING); /* at least one read lock */
+    DBUG_ASSERT(!(l & WRITER)); /* no write lock must have existed */
+  }
+  /** Release an exclusive lock */
+  void write_unlock()
+  {
+    IF_DBUG_ASSERT(auto l=,) lock.fetch_sub(WRITER, std::memory_order_release);
+    DBUG_ASSERT(l & WRITER); /* the write lock must have existed */
+  }
+  /** Try to acquire a shared lock.
+  @return whether the lock was acquired */
+  bool read_trylock() { uint32_t l; return read_trylock(l); }
+  /** Try to acquire an exclusive lock.
+  @return whether the lock was acquired */
+  bool write_trylock()
+  {
+    auto l= UNLOCKED;
+    return lock.compare_exchange_strong(l, WRITER, std::memory_order_acquire,
+                                        std::memory_order_relaxed);
+  }
+
+  /** @return whether an exclusive lock is being held by any thread */
+  bool is_write_locked() const
+  { return !!(lock.load(std::memory_order_relaxed) & WRITER); }
+  /** @return whether a shared lock is being held by any thread */
+  bool is_read_locked() const
+  {
+    auto l= lock.load(std::memory_order_relaxed);
+    return (l & ~WRITER_PENDING) && !(l & WRITER);
+  }
+  /** @return whether any lock is being held by any thread */
+  bool is_locked() const
+  { return (lock.load(std::memory_order_relaxed) & ~WRITER_WAITING) != 0; }
+};