pthreads: reader/writer locks, upgrading read lock to write lock

Question

I'm using read/write locks on Linux and I've found that trying to upgrade a read locked object to a write lock deadlocks.

i.e.

// acquire the read lock in thread 1.
pthread_rwlock_rdlock( &lock );

// make a decision to upgrade the lock in threads 1.
pthread_rwlock_wrlock( &lock ); // this deadlocks as already hold read lock.

I've read the man page and it's quite specific.

The calling thread may deadlock if at the time the call is made it holds the read-write lock (whether a read or write lock).

What is the best way to upgrade a read lock to a write lock in these circumstances.. I don't want to introduce a race on the variable I'm protecting.

Presumably I can create another mutex to encompass the releasing of the read lock and the acquiring of the write lock but then I don't really see the use of read/write locks. I might as well simply use a normal mutex.

Thx

Answer 1

Easiest and safest would be to take the write-lock from the moment you could want to change your data instead of from the moment you're sure you will change it. I know that this will make access to your data a bit more serialized.

I was a bit surprised when reading this question, because I never even considered first taken a read-lock and then upgrading to a write-lock. Well, different situation could need different approaches.

Answer 2

What else than a dead lock do you want in the following scenario?

• thread 1 acquire read lock
• thread 2 acquire read lock
• thread 1 ask to upgrade lock to write
• thread 2 ask to upgrade lock to write

So I'd just release the read lock, acquire the write lock and check again if I've to make the update or not.