I have an interesting problem with deadlocks in an my application. There is an in-memory data store that uses a ReaderWriterLockSlim to synchronize reads and writes. One of the read methods uses Parallel.ForEach to search the store given a set of filters. It's possible that one of the filters requires a constant-time read of same store. Here is the scenario that's producing a a deadlock:
UPDATE: Example code below. Steps updated with actual method calls
Given singleton instance store of ConcreteStoreThatExtendsGenericStore
- Thread1 gets a read lock on the store -
store.Search(someCriteria) - Thread2 attempts to update the store with a write lock -
store.Update()-, blocks behind Thread1 - Thread1 executes Parallel.ForEach against the store to run a set of filters
- Thread3 (spawned by Thread1's Parallel.ForEach) attempts a constant-time read of the store. It tries to get a read lock but is blocked behind Thread2's write lock.
- Thread1 cannot finish because it can't join Thread3. Thread2 can't finish because it's blocked behind Thread1.
Ideally what I'd like to do is not try to acquire a read lock if an ancestor thread of the current thread already has the same lock. Is there any way to do this? Or is there a another/better approach?
public abstract class GenericStore<TKey, TValue>
{
private ReaderWriterLockSlim _lock = new ReaderWriterLockSlim();
private List<IFilter> _filters; //contains instance of ExampleOffendingFilter
protected Dictionary<TKey, TValue> Store { get; private set; }
public void Update()
{
_lock.EnterWriterLock();
//update the store
_lock.ExitWriteLock();
}
public TValue GetByKey(TKey key)
{
TValue value;
//TODO don't enter read lock if current thread
//was started by a thread holding this lock
_lock.EnterReadLock();
value = Store[key];
_lock.ExitReadLock();
return value;
}
public List<TValue> Search(Criteria criteria)
{
List<TValue> matches = new List<TValue>();
//TODO don't enter read lock if current thread
//was started by a thread holding this lock
_lock.EnterReadLock();
Parallel.ForEach(Store.Values, item =>
{
bool isMatch = true;
foreach(IFilter filter in _filters)
{
if (!filter.Check(criteria, item))
{
isMatch = false;
break;
}
}
if (isMatch)
{
lock(matches)
{
matches.Add(item);
}
}
});
_lock.ExitReadLock();
return matches;
}
}
public class ExampleOffendingFilter : IFilter
{
private ConcreteStoreThatExtendsGenericStore _sameStore;
public bool Check(Criteria criteria, ConcreteValueType item)
{
_sameStore.GetByKey(item.SomeRelatedProperty);
return trueOrFalse;
}
}
