{-# LANGUAGE CPP , DeriveDataTypeable , NamedFieldPuns , NoImplicitPrelude , UnicodeSyntax #-} ------------------------------------------------------------------------------- -- | -- Module : Control.Concurrent.ReadWriteLock -- Copyright : (c) 2010-2011 Bas van Dijk & Roel van Dijk -- License : BSD3 (see the file LICENSE) -- Maintainer : Bas van Dijk -- , Roel van Dijk -- -- Multiple-reader, single-writer locks. Used to protect shared resources which -- may be concurrently read, but only sequentially written. -- -- All functions are /exception safe/. Throwing asynchronous exceptions will not -- compromise the internal state of an 'RWLock'. This means it is perfectly safe -- to kill a thread that is blocking on, for example, 'acquireRead'. -- -- See also Java's version: -- -- -- This module is designed to be imported qualified. We suggest importing it -- like: -- -- @ -- import Control.Concurrent.ReadWriteLock ( RWLock ) -- import qualified Control.Concurrent.ReadWriteLock as RWL ( ... ) -- @ -- ------------------------------------------------------------------------------- module Control.Concurrent.ReadWriteLock ( RWLock -- *Creating Read-Write Locks , new , newAcquiredRead , newAcquiredWrite -- *Read access -- **Blocking , acquireRead , releaseRead , withRead , waitRead -- **Non-blocking , tryAcquireRead , tryWithRead -- *Write access -- **Blocking , acquireWrite , releaseWrite , withWrite , waitWrite -- **Non-blocking , tryAcquireWrite , tryWithWrite ) where ------------------------------------------------------------------------------- -- Imports ------------------------------------------------------------------------------- -- from base: import Control.Applicative ( liftA2, liftA3 ) import Control.Concurrent.MVar ( MVar, newMVar, takeMVar, putMVar, swapMVar ) import Control.Exception ( bracket_, onException ) import Control.Monad ( return, return, (>>), when ) import Data.Bool ( Bool(False, True) ) import Data.Eq ( Eq, (==) ) import Data.Function ( ($), on ) import Data.Int ( Int ) import Data.Maybe ( Maybe(Nothing, Just) ) import Data.Typeable ( Typeable ) import Prelude ( String, ($!), succ, pred, error ) import System.IO ( IO ) #if __GLASGOW_HASKELL__ < 700 import Prelude ( fromInteger ) import Control.Monad ( (>>=), fail ) #endif -- from base-unicode-symbols: import Data.Function.Unicode ( (∘) ) import Data.Monoid.Unicode ( (⊕) ) -- from concurrent-extra (this package): import Control.Concurrent.Lock ( Lock ) import qualified Control.Concurrent.Lock as Lock ( new, newAcquired, acquire, tryAcquire, release, wait ) import Utils ( void, mask, mask_ ) ------------------------------------------------------------------------------- -- Read Write Lock ------------------------------------------------------------------------------- {-| Multiple-reader, single-writer lock. Is in one of three states: * \"Free\": Read or write access can be acquired without blocking. * \"Read\": One or more threads have acquired read access. Blocks write access. * \"Write\": A single thread has acquired write access. Blocks other threads from acquiring both read and write access. -} data RWLock = RWLock { state ∷ MVar State , readLock ∷ Lock , writeLock ∷ Lock } deriving Typeable instance Eq RWLock where (==) = (==) `on` state -- | Internal state of the 'RWLock'. data State = Free | Read Int | Write ------------------------------------------------------------------------------- -- * Creating Read-Write Locks ------------------------------------------------------------------------------- -- | Create a new 'RWLock' in the \"free\" state; either read or write access -- can be acquired without blocking. new ∷ IO RWLock new = liftA3 RWLock (newMVar Free) Lock.new Lock.new -- | Create a new 'RWLock' in the \"read\" state; only read can be acquired -- without blocking. newAcquiredRead ∷ IO RWLock newAcquiredRead = liftA3 RWLock (newMVar $ Read 1) Lock.newAcquired Lock.new -- | Create a new 'RWLock' in the \"write\" state; either acquiring read or -- write will block. newAcquiredWrite ∷ IO RWLock newAcquiredWrite = liftA3 RWLock (newMVar Write) Lock.new Lock.newAcquired ------------------------------------------------------------------------------- -- * Read access ------------------------------------------------------------------------------- {-| Acquire the read lock. Blocks if another thread has acquired write access. If @acquireRead@ terminates without throwing an exception the state of the 'RWLock' will be \"read\". Implementation note: Throws an exception when more than (maxBound :: Int) simultaneous threads acquire the read lock. But that is unlikely. -} acquireRead ∷ RWLock → IO () acquireRead (RWLock {state, readLock, writeLock}) = mask_ acqRead where acqRead = do st ← takeMVar state case st of Free → do Lock.acquire readLock putMVar state $ Read 1 Read n → putMVar state ∘ Read $! succ n Write → do putMVar state st Lock.wait writeLock acqRead {-| Try to acquire the read lock; non blocking. Like 'acquireRead', but doesn't block. Returns 'True' if the resulting state is \"read\", 'False' otherwise. -} tryAcquireRead ∷ RWLock → IO Bool tryAcquireRead (RWLock {state, readLock}) = mask_ $ do st ← takeMVar state case st of Free → do Lock.acquire readLock putMVar state $ Read 1 return True Read n → do putMVar state ∘ Read $! succ n return True Write → do putMVar state st return False {-| Release the read lock. If the calling thread was the last one to relinquish read access the state will revert to \"free\". It is an error to release read access to an 'RWLock' which is not in the \"read\" state. -} releaseRead ∷ RWLock → IO () releaseRead (RWLock {state, readLock}) = mask_ $ do st ← takeMVar state case st of Read 1 → do Lock.release readLock putMVar state Free Read n → putMVar state ∘ Read $! pred n _ → do putMVar state st error $ moduleName ⊕ ".releaseRead: already released" {-| A convenience function wich first acquires read access and then performs the computation. When the computation terminates, whether normally or by raising an exception, the read lock is released. -} withRead ∷ RWLock → IO α → IO α withRead = liftA2 bracket_ acquireRead releaseRead {-| A non-blocking 'withRead'. First tries to acquire the lock. If that fails, 'Nothing' is returned. If it succeeds, the computation is performed. When the computation terminates, whether normally or by raising an exception, the lock is released and 'Just' the result of the computation is returned. -} tryWithRead ∷ RWLock → IO α → IO (Maybe α) tryWithRead l a = mask $ \restore → do acquired ← tryAcquireRead l if acquired then do r ← restore a `onException` releaseRead l releaseRead l return $ Just r else return Nothing {-| * When the state is \"write\", @waitRead@ /blocks/ until a call to 'releaseWrite' in another thread changes the state to \"free\". * When the state is \"free\" or \"read\" @waitRead@ returns immediately. @waitRead@ does not alter the state of the lock. Note that @waitRead@ is just a convenience function defined as: @waitRead l = 'mask_' '$' 'acquireRead' l '>>' 'releaseRead' l@ -} waitRead ∷ RWLock → IO () waitRead l = mask_ $ acquireRead l >> releaseRead l ------------------------------------------------------------------------------- -- *Write access ------------------------------------------------------------------------------- {-| Acquire the write lock. Blocks if another thread has acquired either read or write access. If @acquireWrite@ terminates without throwing an exception the state of the 'RWLock' will be \"write\". -} acquireWrite ∷ RWLock → IO () acquireWrite (RWLock {state, readLock, writeLock}) = mask_ acqWrite where acqWrite = do st ← takeMVar state case st of Free → do Lock.acquire writeLock putMVar state Write Read _ → do putMVar state st Lock.wait readLock acqWrite Write → do putMVar state st Lock.acquire writeLock void $ swapMVar state Write {-| Try to acquire the write lock; non blocking. Like 'acquireWrite', but doesn't block. Returns 'True' if the resulting state is \"write\", 'False' otherwise. -} tryAcquireWrite ∷ RWLock → IO Bool tryAcquireWrite (RWLock {state, writeLock}) = mask_ $ do st ← takeMVar state case st of Free → do Lock.acquire writeLock putMVar state Write return True Read _ → do putMVar state st return False Write → do putMVar state st b ← Lock.tryAcquire writeLock when b $ void $ swapMVar state Write return b {-| Release the write lock. If @releaseWrite@ terminates without throwing an exception the state will be \"free\". It is an error to release write access to an 'RWLock' which is not in the \"write\" state. -} releaseWrite ∷ RWLock → IO () releaseWrite (RWLock {state, writeLock}) = mask_ $ do st ← takeMVar state case st of Write → do Lock.release writeLock putMVar state Free _ → do putMVar state st error $ moduleName ⊕ ".releaseWrite: already released" {-| A convenience function wich first acquires write access and then performs the computation. When the computation terminates, whether normally or by raising an exception, the write lock is released. -} withWrite ∷ RWLock → IO α → IO α withWrite = liftA2 bracket_ acquireWrite releaseWrite {-| A non-blocking 'withWrite'. First tries to acquire the lock. If that fails, 'Nothing' is returned. If it succeeds, the computation is performed. When the computation terminates, whether normally or by raising an exception, the lock is released and 'Just' the result of the computation is returned. -} tryWithWrite ∷ RWLock → IO α → IO (Maybe α) tryWithWrite l a = mask $ \restore → do acquired ← tryAcquireWrite l if acquired then do r ← restore a `onException` releaseWrite l releaseWrite l return $ Just r else return Nothing {-| * When the state is \"write\" or \"read\" @waitWrite@ /blocks/ until a call to 'releaseWrite' or 'releaseRead' in another thread changes the state to \"free\". * When the state is \"free\" @waitWrite@ returns immediately. @waitWrite@ does not alter the state of the lock. Note that @waitWrite@ is just a convenience function defined as: @waitWrite l = 'mask_' '$' 'acquireWrite' l '>>' 'releaseWrite' l@ -} waitWrite ∷ RWLock → IO () waitWrite l = mask_ $ acquireWrite l >> releaseWrite l moduleName ∷ String moduleName = "Control.Concurrent.ReadWriteLock"