class Concurrent::ReentrantReadWriteLock
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:
@example
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.
This implementation was inspired by ‘java.util.concurrent.ReentrantReadWriteLock`.
@example
lock = Concurrent::ReentrantReadWriteLock.new lock.with_read_lock { data.retrieve } lock.with_write_lock { data.modify! }
@see docs.oracle.com/javase/7/docs/api/java/util/concurrent/locks/ReentrantReadWriteLock.html java.util.concurrent.ReentrantReadWriteLock
Constants
- MAX_READERS
-
@!visibility private
- MAX_WRITERS
-
@!visibility private
- READER_BITS
-
@!visibility private
- READ_LOCK_MASK
-
@!visibility private
- RUNNING_WRITER
-
@!visibility private
- WAITING_WRITER
-
Used with @Counter: @!visibility private
- WRITER_BITS
-
@!visibility private
- WRITE_LOCK_HELD
-
Used with @HeldCount: @!visibility private
- WRITE_LOCK_MASK
-
@!visibility private
Public Class Methods
Source
# 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
Create a new ‘ReentrantReadWriteLock` in the unlocked state.
Public Instance Methods
Source
# 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 a read lock. If a write lock is held by another thread, will block until it is released.
@return [Boolean] true if the lock is successfully acquired
@raise [Concurrent::ResourceLimitError] if the maximum number of readers
is exceeded.
Source
# 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 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
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.
Source
# 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 a previously acquired read lock.
@return [Boolean] true if the lock is successfully released
Source
# File lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb, line 329 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
Release a previously acquired write lock.
@return [Boolean] true if the lock is successfully released
Source
# 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 to acquire a read lock and return true if we succeed. If it cannot be acquired immediately, return false.
@return [Boolean] true if the lock is successfully acquired
Source
# File lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb, line 310 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
Try to acquire a write lock and return true if we succeed. If it cannot be acquired immediately, return false.
@return [Boolean] true if the lock is successfully acquired
Source
# 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
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.
Source
# 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
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.
Private Instance Methods
Source
# File lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb, line 370 def max_readers?(c = @Counter.value) (c & MAX_READERS) == MAX_READERS end
@!visibility private
Source
# File lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb, line 375 def max_writers?(c = @Counter.value) (c & MAX_WRITERS) == MAX_WRITERS end
@!visibility private
Source
# File lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb, line 345 def running_readers(c = @Counter.value) c & MAX_READERS end
@!visibility private
Source
# File lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb, line 350 def running_readers?(c = @Counter.value) (c & MAX_READERS) > 0 end
@!visibility private
Source
# File lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb, line 355 def running_writer?(c = @Counter.value) c >= RUNNING_WRITER end
@!visibility private
Source
# File lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb, line 365 def waiting_or_running_writer?(c = @Counter.value) c >= WAITING_WRITER end
@!visibility private
Source
# File lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb, line 360 def waiting_writers(c = @Counter.value) (c & MAX_WRITERS) >> READER_BITS end
@!visibility private