Class: Concurrent::ReadWriteLock

Inherits:
Synchronization::Object show all
Defined in:
lib/concurrent/atomic/read_write_lock.rb

Overview

Note:

Do not try to acquire the write lock while already holding a read lock or try to acquire the write lock while you already have it. This will lead to deadlock

Ruby read-write lock implementation

Allows any number of concurrent readers, but only one concurrent writer (And if the “write” lock is taken, any readers who come along will have to wait)

If readers are already active when a writer comes along, the writer will wait for all the readers to finish before going ahead. Any additional readers that come when the writer is already waiting, will also wait (so writers are not starved).

This implementation is based on ‘java.util.concurrent.ReentrantReadWriteLock`.

Examples:

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

See Also:

Instance Method Summary collapse

Methods inherited from Synchronization::Object

attr_atomic, attr_volatile, ensure_safe_initialization_when_final_fields_are_present, new, safe_initialization!, safe_initialization?, volatile_cas_fields

Constructor Details

#initialize ⇒ ReadWriteLock

Create a new ‘ReadWriteLock` in the unlocked state.



58
59
60
61
62
63
 
# File 'lib/concurrent/atomic/read_write_lock.rb', line 58

def initialize
  super()
  @Counter   = AtomicFixnum.new(0) # single integer which represents lock state
  @ReadLock  = Synchronization::Lock.new
  @WriteLock = Synchronization::Lock.new
end

Instance Method Details

#acquire_read_lock ⇒ Boolean

Acquire a read lock. If a write lock has been acquired will block until it is released. Will not block if other read locks have been acquired.

Returns:

  • (Boolean) —

    true if the lock is successfully acquired

Raises:



110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
 
# File 'lib/concurrent/atomic/read_write_lock.rb', line 110

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

    # If a writer is waiting when we first queue up, we need to wait
    if waiting_writer?(c)
      @ReadLock.wait_until { !waiting_writer? }

      # 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)
          @ReadLock.wait_until { !running_writer? }
        else
          return if @Counter.compare_and_set(c, c+1)
        end
      end
    else
      break if @Counter.compare_and_set(c, c+1)
    end
  end
  true
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:



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
 
# File 'lib/concurrent/atomic/read_write_lock.rb', line 159

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

    if c == 0 # no readers OR writers running
      # if we successfully swap the RUNNING_WRITER bit on, then we can go ahead
      break if @Counter.compare_and_set(0, RUNNING_WRITER)
    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, OR another writer could increment
        @WriteLock.wait_until do
          # So we have to do another check inside the synchronized section
          # If a writer OR reader is running, then go to sleep
          c = @Counter.value
          !running_writer?(c) && !running_readers?(c)
        end

        # 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
        break if !running_writer?(c) && !running_readers?(c) && @Counter.compare_and_set(c, c+RUNNING_WRITER-WAITING_WRITER)
      end
      break
    end
  end
  true
end

#has_waiters? ⇒ Boolean

Queries whether any threads are waiting to acquire the read or write lock.

Returns:

  • (Boolean) —

    true if any threads are waiting for a lock else false



212
213
214
 
# File 'lib/concurrent/atomic/read_write_lock.rb', line 212

def has_waiters?
  waiting_writer?(@Counter.value)
end

#release_read_lock ⇒ Boolean

Release a previously acquired read lock.

Returns:

  • (Boolean) —

    true if the lock is successfully released



139
140
141
142
143
144
145
146
147
148
149
150
151
 
# File 'lib/concurrent/atomic/read_write_lock.rb', line 139

def release_read_lock
  while true
    c = @Counter.value
    if @Counter.compare_and_set(c, c-1)
      # If one or more writers were waiting, and we were the last reader, wake a writer up
      if waiting_writer?(c) && running_readers(c) == 1
        @WriteLock.signal
      end
      break
    end
  end
  true
end

#release_write_lock ⇒ Boolean

Release a previously acquired write lock.

Returns:

  • (Boolean) —

    true if the lock is successfully released



195
196
197
198
199
200
 
# File 'lib/concurrent/atomic/read_write_lock.rb', line 195

def release_write_lock
  c = @Counter.update { |counter| counter-RUNNING_WRITER }
  @ReadLock.broadcast
  @WriteLock.signal if waiting_writers(c) > 0
  true
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:



74
75
76
77
78
79
80
81
82
 
# File 'lib/concurrent/atomic/read_write_lock.rb', line 74

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:



93
94
95
96
97
98
99
100
101
 
# File 'lib/concurrent/atomic/read_write_lock.rb', line 93

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

#write_locked? ⇒ Boolean

Queries if the write lock is held by any thread.

Returns:

  • (Boolean) —

    true if the write lock is held else false`



205
206
207
 
# File 'lib/concurrent/atomic/read_write_lock.rb', line 205

def write_locked?
  @Counter.value >= RUNNING_WRITER
end