Class: Concurrent::ReentrantReadWriteLock — Concurrent

Class List Method List File List

# Class: Concurrent::ReentrantReadWriteLock

Inherits:
  Synchronization::Object
  - Object
  - Synchronization::Object
  - Concurrent::ReentrantReadWriteLock
  show all
Defined in:
  lib/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.

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

## Constant Summary

## Instance Method Summary (collapse)

- - (Boolean) acquire_read_lock

  Acquire a read lock.

- - (Boolean) acquire_write_lock

  Acquire a write lock.

- - (ReentrantReadWriteLock) initialize constructor

  Create a new `ReentrantReadWriteLock` in the unlocked state.

- - (Boolean) release_read_lock

  Release a previously acquired read lock.

- - (Boolean) release_write_lock

  Release a previously acquired write lock.

- - (Boolean) try_read_lock

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

- - (Boolean) try_write_lock

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

- - (Object) with_read_lock { ... }

  Execute a block operation within a read lock.

- - (Object) with_write_lock { ... }

  Execute a block operation within a write lock.

## Constructor Details

### - (`ReentrantReadWriteLock`) initialize

Create a new `ReentrantReadWriteLock` in the unlocked state.

    105
    106
    107
    108
    109
    110
    111
    # File 'lib/concurrent/atomic/reentrant_read_write_lock.rb', line 105

    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

### - (`Boolean`) acquire_read_lock

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.

    158
    159
    160
    161
    162
    163
    164
    165
    166
    167
    168
    169
    170
    171
    172
    173
    174
    175
    176
    177
    178
    179
    180
    181
    182
    183
    184
    185
    186
    187
    188
    189
    190
    191
    192
    193
    194
    195
    196
    197
    198
    199
    200
    201
    202
    203
    204
    205
    # File 'lib/concurrent/atomic/reentrant_read_write_lock.rb', line 158

    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

### - (`Boolean`) acquire_write_lock

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.

    253
    254
    255
    256
    257
    258
    259
    260
    261
    262
    263
    264
    265
    266
    267
    268
    269
    270
    271
    272
    273
    274
    275
    276
    277
    278
    279
    280
    281
    282
    283
    284
    285
    286
    287
    288
    289
    290
    291
    292
    293
    294
    295
    296
    297
    298
    299
    300
    301
    302
    # File 'lib/concurrent/atomic/reentrant_read_write_lock.rb', line 253

    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 c == held
          # If we successfully swap the RUNNING_WRITER bit on, then we can go ahead
          if @Counter.compare_and_set(c, c+RUNNING_WRITER)
            @HeldCount.value = held + WRITE_LOCK_HELD
            return true
          end
        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

### - (`Boolean`) release_read_lock

Release a previously acquired read lock.

Returns:

- (`Boolean`) —

  true if the lock is successfully released

    232
    233
    234
    235
    236
    237
    238
    239
    240
    241
    242
    243
    244
    245
    # File 'lib/concurrent/atomic/reentrant_read_write_lock.rb', line 232

    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

### - (`Boolean`) release_write_lock

Release a previously acquired write lock.

Returns:

- (`Boolean`) —

  true if the lock is successfully released

    327
    328
    329
    330
    331
    332
    333
    334
    335
    336
    337
    338
    # File 'lib/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

### - (`Boolean`) try_read_lock

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

    211
    212
    213
    214
    215
    216
    217
    218
    219
    220
    221
    222
    223
    224
    225
    226
    227
    # File 'lib/concurrent/atomic/reentrant_read_write_lock.rb', line 211

    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

### - (`Boolean`) try_write_lock

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

    308
    309
    310
    311
    312
    313
    314
    315
    316
    317
    318
    319
    320
    321
    322
    # File 'lib/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

### - (`Object`) with_read_lock { ... }

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.

    122
    123
    124
    125
    126
    127
    128
    129
    130
    # File 'lib/concurrent/atomic/reentrant_read_write_lock.rb', line 122

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

### - (`Object`) with_write_lock { ... }

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.

    141
    142
    143
    144
    145
    146
    147
    148
    149
    # File 'lib/concurrent/atomic/reentrant_read_write_lock.rb', line 141

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