READER/WRITER LOCK in Visual C#

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READER/WRITER LOCK
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As you can see in listing 9.3, it is similar to AcquireReadLock in that it accepts a timeout parameter. The AcquireWriteLock method is, obviously, used to acquire a write lock.
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AcquireWriterLock
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AcquireWriterLock is a method on the ReaderWriterLock class that allows a thread to request ownership of a write lock. It accepts a timeout parameter and throws an ApplicationException if the lock cannot be acquired in the specified time period.
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If the write lock cannot be acquired within the specified duration, an exception is raised. Figure 9.3 shows the relationship between a read lock and a write lock. At any given point a thread cannot have a write lock and some other thread have a read lock on the same instance of the ReaderWriterLock class. When a thread wishes to acquire a write lock, it calls AcquireWriteLock. It then must wait until all threads that currently have read locks release them. Once all threads have released the read locks, the requesting thread is granted its write lock. While that thread has a write lock, no other threads will be able to acquire a read or write lock.
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Figure 9.3 When a write lock has been granted, no thread will be granted a read or write lock until it is released.
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ACQUIRING A WRITER LOCK FROM A READERWRITERLOCK
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IsWriterLockHeld
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IsWriterLockHeld is a property of the ReaderWriterLock class that allows a thread to determine if it has acquired a write lock on an instance of the ReaderWriterLock class. If the thread currently owns a write lock, true is returned.
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To release a write lock, a thread uses the ReleaseWriterLock method of the ReaderWriterLock class. If the thread does not own the lock, an ApplicationException is raised with the message Attempt to release mutex not owned by caller. Once the thread has released its write lock, other threads are able to acquire their desired locks. This ensures that the data a thread is viewing doesn t change while it is looking at it. Care should be taken to ensure that a thread does not modify shared data unless it currently owns a write lock. 9.2.2 UpgradeToWriterLock There are times when it s unclear if the lock required will be a reader or a writer. For example, in the auction simulation, in order to determine if a new bid is higher than the existing bid we must first look at what the current bid is (listing 9.4). Once we ve examined the current bid, we can see if the new bid is higher.
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Listing 9.4 Checks to see if the new bid is higher than the existing (C#)
public void Bid(decimal Amount, string BiddersName) { if (ItemLock.IsWriterLockHeld) { throw new Exception("Writer Lock held before acquire"); } if (ItemLock.IsReaderLockHeld) { throw new Exception("Reader Lock held before acquire"); } ItemLock.AcquireReaderLock(Timeout.Infinite); Initially acquire try a reader lock { if (DateTime.Now > TheAuctionEnds) { throw new Exception("Auction has ended"); } See if we need to if (Amount > TheCurrentPrice) acquire a writer lock { ItemLock.UpgradeToWriterLock(60000); Upgrade to if (!ItemLock.IsWriterLockHeld) a writer lock { throw new Exception("Writer Lock not held after upgrade"); } if (Amount > TheCurrentPrice) Check to see if { we re still the TheCurrentPrice = Amount; highest bidder TheBiddersName=BiddersName;
READER/WRITER LOCK
} else { throw new Exception("Bid not higher than current price"); } } else { throw new Exception("Bid not higher than current price"); } } ReleaseReaderLock finally releases both { Reader and Writer ItemLock.ReleaseReaderLock(); if (ItemLock.IsWriterLockHeld) { throw new Exception("Writer Lock still held after release"); } if (ItemLock.IsReaderLockHeld) { throw new Exception("Reader Lock still held after release"); } } }
In listing 9.4, it s unclear if a writer lock is needed until the bid amount is compared to the current price. In the last section, we dealt with this by acquiring a write lock. A more optimal solution is to acquire a read lock and determine if a write lock is required. If it is, we call UpgradeToWriterLock. The advantage is that we only require a write lock when it is needed. Since write locks keep all reader locks from accessing data, using them unnecessarily results in reduced performance. Be careful when upgrading from a read to a write lock. There is a relatively small chance that during the transition from read to write some other pending write request may change the value. Figure 9.4 presents a graphical version of the logic involved.
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