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`.
@example
lock = Concurrent::ReadWriteLock.new lock.with_read_lock { data.retrieve } lock.with_write_lock { data.modify! }
@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
@see docs.oracle.com/javase/7/docs/api/java/util/concurrent/locks/ReentrantReadWriteLock.html java.util.concurrent.ReentrantReadWriteLock
@!visibility private
@!visibility private
@!visibility private
@!visibility private
Create a new `ReadWriteLock` in the unlocked state.
# File lib/concurrent/atomic/read_write_lock.rb, line 56 def initialize @Counter = AtomicReference.new(0) # single integer which represents lock state @ReadLock = Synchronization::Lock.new @WriteLock = Synchronization::Lock.new ensure_ivar_visibility! super() end
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.
@return [Boolean] true if the lock is successfully acquired
@raise [Concurrent::ResourceLimitError] if the maximum number of readers
is exceeded.
# File lib/concurrent/atomic/read_write_lock.rb, line 109 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_swap(c, c+1) end end else break if @Counter.compare_and_swap(c, c+1) end end true end
Acquire a write lock. Will block and wait for all active readers and writers.
@return [Boolean] true if the lock is successfully acquired
@raise [Concurrent::ResourceLimitError] if the maximum number of writers
is exceeded.
# File lib/concurrent/atomic/read_write_lock.rb, line 158 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_swap(0, RUNNING_WRITER) elsif @Counter.compare_and_swap(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_swap(c, c+RUNNING_WRITER-WAITING_WRITER) end break end end true end
Queries whether any threads are waiting to acquire the read or write lock.
@return [Boolean] true if any threads are waiting for a lock else false
# File lib/concurrent/atomic/read_write_lock.rb, line 211 def has_waiters? waiting_writer?(@Counter.value) end
Release a previously acquired read lock.
@return [Boolean] true if the lock is successfully released
# File lib/concurrent/atomic/read_write_lock.rb, line 138 def release_read_lock while true c = @Counter.value if @Counter.compare_and_swap(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 a previously acquired write lock.
@return [Boolean] true if the lock is successfully released
# File lib/concurrent/atomic/read_write_lock.rb, line 194 def release_write_lock c = @Counter.update { |c| c-RUNNING_WRITER } @ReadLock.broadcast @WriteLock.signal if waiting_writers(c) > 0 true end
Execute a block operation within a read lock.
@yield the task to be performed within the lock.
@return [Object] the result of the block operation.
@raise [ArgumentError] when no block is given. @raise [Concurrent::ResourceLimitError] if the maximum number of readers
is exceeded.
# File lib/concurrent/atomic/read_write_lock.rb, line 73 def with_read_lock raise ArgumentError.new('no block given') unless block_given? acquire_read_lock begin yield ensure release_read_lock end end
Execute a block operation within a write lock.
@yield the task to be performed within the lock.
@return [Object] the result of the block operation.
@raise [ArgumentError] when no block is given. @raise [Concurrent::ResourceLimitError] if the maximum number of readers
is exceeded.
# File lib/concurrent/atomic/read_write_lock.rb, line 92 def with_write_lock raise ArgumentError.new('no block given') unless block_given? acquire_write_lock begin yield ensure release_write_lock end end
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