class Garcon::ReadWriteLock
read-write Lock for cooking safety.
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).
@example
lock = Garcon::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
Constants
- MAX_READERS
@!visibility private
- MAX_WRITERS
@!visibility private
- RUNNING_WRITER
@!visibility private
- WAITING_WRITER
@!visibility private
Public Class Methods
Create a new ‘ReadWriteLock` in the unlocked state.
# File lib/garcon/task/read_write_lock.rb, line 72 def initialize @counter = AtomicMutex.new(0) # represents lock state @reader_q = ConditionVariable.new # queue for waiting readers @reader_mutex = Mutex.new # to protect reader queue @writer_q = ConditionVariable.new # queue for waiting writers @writer_mutex = Mutex.new # to protect writer queue end
Public Instance Methods
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 [Garcon::ResourceLimitError]
If the maximum number of readers is exceeded.
# File lib/garcon/task/read_write_lock.rb, line 134 def acquire_read_lock while(true) c = @counter.value raise ResourceLimitError, 'Too many reader threads' if max_readers?(c) if waiting_writer?(c) @reader_mutex.synchronize do @reader_q.wait(@reader_mutex) if waiting_writer? end while(true) c = @counter.value if running_writer?(c) @reader_mutex.synchronize do @reader_q.wait(@reader_mutex) if running_writer? end 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 [Garcon::ResourceLimitError]
If the maximum number of writers is exceeded.
# File lib/garcon/task/read_write_lock.rb, line 188 def acquire_write_lock while(true) c = @counter.value raise ResourceLimitError, 'Too many writer threads' if max_writers?(c) if c == 0 break if @counter.compare_and_swap(0,RUNNING_WRITER) elsif @counter.compare_and_swap(c,c+WAITING_WRITER) while(true) @writer_mutex.synchronize do c = @counter.value if running_writer?(c) || running_readers?(c) @writer_q.wait(@writer_mutex) end end 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/garcon/task/read_write_lock.rb, line 263 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/garcon/task/read_write_lock.rb, line 166 def release_read_lock while(true) c = @counter.value if @counter.compare_and_swap(c,c-1) if waiting_writer?(c) && running_readers(c) == 1 @writer_mutex.synchronize { @writer_q.signal } end break end end true end
Release a previously acquired write lock.
@return [Boolean]
True if the lock is successfully released.
# File lib/garcon/task/read_write_lock.rb, line 219 def release_write_lock while(true) c = @counter.value if @counter.compare_and_swap(c,c-RUNNING_WRITER) @reader_mutex.synchronize { @reader_q.broadcast } if waiting_writers(c) > 0 @writer_mutex.synchronize { @writer_q.signal } end break end end true end
Returns a string representing obj. Includes the current reader and writer counts.
# File lib/garcon/task/read_write_lock.rb, line 236 def to_s c = @counter.value s = if running_writer?(c) "1 writer running, " elsif running_readers(c) > 0 "#{running_readers(c)} readers running, " else "" end "#<ReadWriteLock:#{object_id.to_s(16)} #{s}#{waiting_writers(c)} writers waiting>" end
Execute a block operation within a read lock.
@return [Object]
The result of the block operation.
@yield the task to be performed within the lock.
@raise [ArgumentError]
When no block is given.
@raise [Garcon::ResourceLimitError]
If the maximum number of readers is exceeded.
# File lib/garcon/task/read_write_lock.rb, line 93 def with_read_lock raise ArgumentError, '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.
@return [Object] the result of the block operation.
@yield the task to be performed within the lock.
@raise [ArgumentError]
When no block is given.
@raise [Garcon::ResourceLimitError]
If the maximum number of readers is exceeded.
# File lib/garcon/task/read_write_lock.rb, line 115 def with_write_lock raise ArgumentError, 'no block given' unless block_given? acquire_write_lock begin yield ensure release_write_lock end end
Queries if the write lock is held by any thread.
@return [Boolean]
True if the write lock is held else false.
# File lib/garcon/task/read_write_lock.rb, line 254 def write_locked? @counter.value >= RUNNING_WRITER end