# Class: Concurrent::ReentrantReadWriteLock

Inherits:
  Synchronization::Object
  - Object
  - Synchronization::Object
  - Concurrent::ReentrantReadWriteLock
  show all

Defined in:
  lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb

## Overview

Re-entrant read-write lock implementation

Allows any number of concurrent readers, but only one concurrent writer (And while the "write" lock is taken, no read locks can be obtained either. Hence, the write lock can also be called an "exclusive" lock.)

If another thread has taken a read lock, any thread which wants a write lock will block until all the readers release their locks. However, once a thread starts waiting to obtain a write lock, any additional readers that come along will also wait (so writers are not starved).

A thread can acquire both a read and write lock at the same time. A thread can also acquire a read lock OR a write lock more than once. Only when the read (or write) lock is released as many times as it was acquired, will the thread actually let it go, allowing other threads which might have been waiting to proceed. Therefore the lock can be upgraded by first acquiring read lock and then write lock and that the lock can be downgraded by first having both read and write lock a releasing just the write lock.

If both read and write locks are acquired by the same thread, it is not strictly necessary to release them in the same order they were acquired. In other words, the following code is legal:

This implementation was inspired by `java.util.concurrent.ReentrantReadWriteLock`.

Examples:

    lock = Concurrent::ReentrantReadWriteLock.new
    lock.acquire_write_lock
    lock.acquire_read_lock
    lock.release_write_lock
    # At this point, the current thread is holding only a read lock, not a write
    # lock. So other threads can take read locks, but not a write lock.
    lock.release_read_lock
    # Now the current thread is not holding either a read or write lock, so
    # another thread could potentially acquire a write lock.

    lock = Concurrent::ReentrantReadWriteLock.new
    lock.with_read_lock  { data.retrieve }
    lock.with_write_lock { data.modify! }

See Also:

- java.util.concurrent.ReentrantReadWriteLock

## Instance Method Summary collapse

- #acquire_read_lock ⇒ Boolean

  Acquire a read lock.

- #acquire_write_lock ⇒ Boolean

  Acquire a write lock.

- #initialize ⇒ ReentrantReadWriteLock constructor

  Create a new `ReentrantReadWriteLock` in the unlocked state.

- #release_read_lock ⇒ Boolean

  Release a previously acquired read lock.

- #release_write_lock ⇒ Boolean

  Release a previously acquired write lock.

- #try_read_lock ⇒ Boolean

  Try to acquire a read lock and return true if we succeed.

- #try_write_lock ⇒ Boolean

  Try to acquire a write lock and return true if we succeed.

- #with_read_lock { ... } ⇒ Object

  Execute a block operation within a read lock.

- #with_write_lock { ... } ⇒ Object

  Execute a block operation within a write lock.

## Constructor Details

### #initialize ⇒ `ReentrantReadWriteLock`

Create a new `ReentrantReadWriteLock` in the unlocked state.

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    # File 'lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb', line 107

    def initialize
      super()
      @Counter    = AtomicFixnum.new(0)       # single integer which represents lock state
      @ReadQueue  = Synchronization::Lock.new # used to queue waiting readers
      @WriteQueue = Synchronization::Lock.new # used to queue waiting writers
      @HeldCount  = ThreadLocalVar.new(0)     # indicates # of R & W locks held by this thread
    end

## Instance Method Details

### #acquire_read_lock ⇒ `Boolean`

Acquire a read lock. If a write lock is held by another thread, will block until it is released.

Returns:

- (`Boolean`) —

  true if the lock is successfully acquired

Raises:

- (`Concurrent::ResourceLimitError`) —

  if the maximum number of readers is exceeded.

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    # File 'lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb', line 160

    def acquire_read_lock
      if (held = @HeldCount.value) > 0
        # If we already have a lock, there's no need to wait
        if held & READ_LOCK_MASK == 0
          # But we do need to update the counter, if we were holding a write
          #   lock but not a read lock
          @Counter.update { |c| c + 1 }
        end
        @HeldCount.value = held + 1
        return true
      end

      while true
        c = @Counter.value
        raise ResourceLimitError.new('Too many reader threads') if max_readers?(c)

        # If a writer is waiting OR running when we first queue up, we need to wait
        if waiting_or_running_writer?(c)
          # Before going to sleep, check again with the ReadQueue mutex held
          @ReadQueue.synchronize do
            @ReadQueue.ns_wait if waiting_or_running_writer?
          end
          # Note: the above 'synchronize' block could have used #wait_until,
          #   but that waits repeatedly in a loop, checking the wait condition
          #   each time it wakes up (to protect against spurious wakeups)
          # But we are already in a loop, which is only broken when we successfully
          #   acquire the lock! So we don't care about spurious wakeups, and would
          #   rather not pay the extra overhead of using #wait_until

          # After a reader has waited once, they are allowed to "barge" ahead of waiting writers
          # But if a writer is *running*, the reader still needs to wait (naturally)
          while true
            c = @Counter.value
            if running_writer?(c)
              @ReadQueue.synchronize do
                @ReadQueue.ns_wait if running_writer?
              end
            elsif @Counter.compare_and_set(c, c+1)
              @HeldCount.value = held + 1
              return true
            end
          end
        elsif @Counter.compare_and_set(c, c+1)
          @HeldCount.value = held + 1
          return true
        end
      end
    end

### #acquire_write_lock ⇒ `Boolean`

Acquire a write lock. Will block and wait for all active readers and writers.

Returns:

- (`Boolean`) —

  true if the lock is successfully acquired

Raises:

- (`Concurrent::ResourceLimitError`) —

  if the maximum number of writers is exceeded.

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    # File 'lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb', line 255

    def acquire_write_lock
      if (held = @HeldCount.value) >= WRITE_LOCK_HELD
        # if we already have a write (exclusive) lock, there's no need to wait
        @HeldCount.value = held + WRITE_LOCK_HELD
        return true
      end

      while true
        c = @Counter.value
        raise ResourceLimitError.new('Too many writer threads') if max_writers?(c)

        # To go ahead and take the lock without waiting, there must be no writer
        #   running right now, AND no writers who came before us still waiting to
        #   acquire the lock
        # Additionally, if any read locks have been taken, we must hold all of them
        if held > 0 && @Counter.compare_and_set(1, c+RUNNING_WRITER)
          # If we are the only one reader and successfully swap the RUNNING_WRITER bit on, then we can go ahead
          @HeldCount.value = held + WRITE_LOCK_HELD
          return true
        elsif @Counter.compare_and_set(c, c+WAITING_WRITER)
          while true
            # Now we have successfully incremented, so no more readers will be able to increment
            #   (they will wait instead)
            # However, readers OR writers could decrement right here
            @WriteQueue.synchronize do
              # So we have to do another check inside the synchronized section
              # If a writer OR another reader is running, then go to sleep
              c = @Counter.value
              @WriteQueue.ns_wait if running_writer?(c) || running_readers(c) != held
            end
            # Note: if you are thinking of replacing the above 'synchronize' block
            # with #wait_until, read the comment in #acquire_read_lock first!

            # We just came out of a wait
            # If we successfully turn the RUNNING_WRITER bit on with an atomic swap,
            #   then we are OK to stop waiting and go ahead
            # Otherwise go back and wait again
            c = @Counter.value
            if !running_writer?(c) &&
               running_readers(c) == held &&
               @Counter.compare_and_set(c, c+RUNNING_WRITER-WAITING_WRITER)
              @HeldCount.value = held + WRITE_LOCK_HELD
              return true
            end
          end
        end
      end
    end

### #release_read_lock ⇒ `Boolean`

Release a previously acquired read lock.

Returns:

- (`Boolean`) —

  true if the lock is successfully released

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    # File 'lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb', line 234

    def release_read_lock
      held = @HeldCount.value = @HeldCount.value - 1
      rlocks_held = held & READ_LOCK_MASK
      if rlocks_held == 0
        c = @Counter.update { |counter| counter - 1 }
        # If one or more writers were waiting, and we were the last reader, wake a writer up
        if waiting_or_running_writer?(c) && running_readers(c) == 0
          @WriteQueue.signal
        end
      elsif rlocks_held == READ_LOCK_MASK
        raise IllegalOperationError, "Cannot release a read lock which is not held"
      end
      true
    end

### #release_write_lock ⇒ `Boolean`

Release a previously acquired write lock.

Returns:

- (`Boolean`) —

  true if the lock is successfully released

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    # File 'lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb', line 327

    def release_write_lock
      held = @HeldCount.value = @HeldCount.value - WRITE_LOCK_HELD
      wlocks_held = held & WRITE_LOCK_MASK
      if wlocks_held == 0
        c = @Counter.update { |counter| counter - RUNNING_WRITER }
        @ReadQueue.broadcast
        @WriteQueue.signal if waiting_writers(c) > 0
      elsif wlocks_held == WRITE_LOCK_MASK
        raise IllegalOperationError, "Cannot release a write lock which is not held"
      end
      true
    end

### #try_read_lock ⇒ `Boolean`

Try to acquire a read lock and return true if we succeed. If it cannot be acquired immediately, return false.

Returns:

- (`Boolean`) —

  true if the lock is successfully acquired

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    # File 'lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb', line 213

    def try_read_lock
      if (held = @HeldCount.value) > 0
        if held & READ_LOCK_MASK == 0
          # If we hold a write lock, but not a read lock...
          @Counter.update { |c| c + 1 }
        end
        @HeldCount.value = held + 1
        return true
      else
        c = @Counter.value
        if !waiting_or_running_writer?(c) && @Counter.compare_and_set(c, c+1)
          @HeldCount.value = held + 1
          return true
        end
      end
      false
    end

### #try_write_lock ⇒ `Boolean`

Try to acquire a write lock and return true if we succeed. If it cannot be acquired immediately, return false.

Returns:

- (`Boolean`) —

  true if the lock is successfully acquired

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    # File 'lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb', line 308

    def try_write_lock
      if (held = @HeldCount.value) >= WRITE_LOCK_HELD
        @HeldCount.value = held + WRITE_LOCK_HELD
        return true
      else
        c = @Counter.value
        if !waiting_or_running_writer?(c) &&
           running_readers(c) == held &&
           @Counter.compare_and_set(c, c+RUNNING_WRITER)
           @HeldCount.value = held + WRITE_LOCK_HELD
          return true
        end
      end
      false
    end

### #with_read_lock { ... } ⇒ `Object`

Execute a block operation within a read lock.

Yields:

- the task to be performed within the lock.

Returns:

- (`Object`) —

  the result of the block operation.

Raises:

- (`ArgumentError`) —

  when no block is given.

- (`Concurrent::ResourceLimitError`) —

  if the maximum number of readers is exceeded.

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    # File 'lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb', line 124

    def with_read_lock
      raise ArgumentError.new('no block given') unless block_given?
      acquire_read_lock
      begin
        yield
      ensure
        release_read_lock
      end
    end

### #with_write_lock { ... } ⇒ `Object`

Execute a block operation within a write lock.

Yields:

- the task to be performed within the lock.

Returns:

- (`Object`) —

  the result of the block operation.

Raises:

- (`ArgumentError`) —

  when no block is given.

- (`Concurrent::ResourceLimitError`) —

  if the maximum number of readers is exceeded.

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    # File 'lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb', line 143

    def with_write_lock
      raise ArgumentError.new('no block given') unless block_given?
      acquire_write_lock
      begin
        yield
      ensure
        release_write_lock
      end
    end