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Data races are a concern only when shared data is modified. Multiple threads reading the shared data do not present a problem. Read-only data does not, therefore, need protec-tion with some kind of lock.
However, sometimes data that is typically read-only needs to be updated. A readers-writer lock (or multiple-reader lock) allows many threads to read the shared data but can then lock the readers threads out to allow one thread to acquire a writer lock to modify the data.
A writer cannot acquire the write lock until all the readers have released their reader locks. For this reason, the locks tend to be biased toward writers; as soon as one is queued, the lock stops allowing further readers to enter. This action causes the number of readers holding the lock to diminish and will eventually allow the writer to get exclu-sive access to the lock.
The code snippet in Listing 4.9 shows how a readers-writer lock might be used. Most threads will be calling the routine readData() to return the value from a particular pair of cells. Once a thread has a reader lock, they can read the value of the pair of cells, before releasing the reader lock.
To modify the data, a thread needs to acquire a writer lock. This will stop any reader threads from acquiring a reader lock. Eventually all the reader threads will have released their lock, and only at that point does the writer thread actually acquire the lock and is allowed to update the data.
Listing 4.9 Using a Readers-Writer Lock
int readData( int cell1, int cell2 )
acquireReaderLock( &lock );
int result = data[cell] + data[cell2];
releaseReaderLock( &lock );
void writeData( int cell1, int cell2, int value )
acquireWriterLock( &lock );
data[cell1] += value;
data[cell2] -= value;
releaseWriterLock( &lock );
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