Re: Can a thread write-lock a rwlock that it already has read-locked?

[Branko Čibej]

David Barrett wrote:
Anyway, that's my goal, and while I'd love any feedback on my approach
(especially to point out any gaping logic holes I've overlooked), I think
I'm all set.
 

I think your algorithm is a deadlock waiting to happen. Consider the 
following situation:

   thraad A acquires read lock
   thread B acquires read lock
   thread C acquires read lock
   thread A requests write lock (blocks)
   thread B requests write lock (blocks)
   thread C releases read lock
At this point, threads A and B are both waiting for other readers to 
release their locks, and there are no writers yet. So they're waiting 
for each other. Even if you allow a single thread to own a write and any 
number of read locks simultaneously, you can't avoid this kind of 
deadlock unless each thread releases all its read locks before acquiring 
a write lock. The best you can do is to fail a write-lock request if 
another thread is waiting for the write lock, but to avoid the deadlock 
the thread whose write-lock failed would still have to release all its 
read locks before retrying the request.

Again, you end up with the algorithm I showed in my previous post, 
except that the code gets even more complex.

All in all, I'd advise not trying to invent a "better" way of locking, 
but to use proven methods. Locking is notoriously tricky.

-- Brane