Question

I have a question about pthread_wrlock_wrlock and pthread_wrlock_rdlock:
Are they not implemented as spin locks??

I tried this in my code:

pthread_rwlock_t rwlock = PTHREAD_RWLOCK_INITIALIZER;
pthread_rwlock_rdlock (&rwlock);
pthread_rwlock_wrlock (&rwlock);

=> this causes deadlock as I expected

However, I don’t understand why the revered one doesn’t cause deadlock:

pthread_rwlock_t rwlock = PTHREAD_RWLOCK_INITIALIZER;
pthread_rwlock_wrlock (&rwlock); // return 0
pthread_rwlock_rdlock (&rwlock); // return 35

If pthread_rwlock_rdlock spins, why does it return failure rather than spin?

Answer 1

Because it is the same thread, it would produce a deadlock hence the return code EDEADLK which is definitely not what you want.

The first case looks like a defect as it should also return EDEADLK. Blocking should only occur in separately threads.