Question

Disclaimer: I’m not very good at Java and just comparing read/writer locks between C# and Java to understand this topic better & decisions behind both implementations.

There is JavaDoc about ReentrantReadWriteLock. It states the following about upgrade/downgrade for locks:

• Lock downgrading … However, upgrading from a read lock to the write lock is not possible.

It also has the following example that shows manual upgrade from read lock to write lock:

 // Here is a code sketch showing how to exploit reentrancy 
 // to perform lock downgrading after updating a cache

 void processCachedData() {
 rwl.readLock().lock();
 if (!cacheValid) {
    // upgrade lock manually
    #1: rwl.readLock().unlock();   // must unlock first to obtain writelock
    #2: rwl.writeLock().lock();
    if (!cacheValid) { // recheck
       ...
    }
   ...
 }
 use(data);
 rwl.readLock().unlock();

Does it mean that actually the sample from above may not behave correctly in some cases – I mean there is no lock between lines #1 & #2 and underlying structure is exposed to changes from other threads. So it can not be considered as the correct way to upgrade the lock or do I miss something here?

Answer 1

Yes, you are right. But this code handles the situation by calling if (!cacheValid) { // recheck again after it acquired the write lock.