Can I use multiple omp_lock_t's in OpenMP?

Question

I assumed that I would be able to create 2 different omp_lock_t's, and lock them independently of each other. I tested the following section of code with gcc 4.4 and gcc 4.6.1 and got the same output:

omp_lock_t lockA;
omp_lock_t lockB;
omp_init_lock(&lockA);
omp_init_lock(&lockB);

std::cout << "Lock B is: " omp_test_lock(&lockB) << "\n";
omp_set_lock(&lockA);
std::cout << "Lock A set\n";
std::cout << "Lock B is: " omp_test_lock(&lockB) << "\n";

omp_set_lock(&lockB);
...

This code produces the following output:

Lock B is: 1
Lock A set
Lock B is: 0

then it deadlocks on the omp_set_lock(&lockB) attempt.

Is it not possible to create two different locks and use them independently? If it is possible, what is the correct way to set up these locks?

Thanks

Answer 1

The code works as expected. I’m assuming that you have a misunderstanding about what omp_test_lock does (especially since the name is admittedly very misleading).

omp_test_lock tries to acquire a lock. But unless omp_set_lock, it doesn’t block until it has successfully acquired the lock. But that’s the only difference. Notably, after the first “test” (which isn’t just a test), lockB is acquired, that’s why the second “test” fails, and why the subsequent omp_set_lock(&lockB) deadlocks.

And just to make this clear: yes, you can use multiple locks.

omp_lock_t lockA;
omp_lock_t lockB;
omp_init_lock(&lockA);
omp_init_lock(&lockB);

omp_set_lock(&lockA);
std::cout << "Lock A set\n";
omp_set_lock(&lockB);
std::cout << "Lock B set\n";
…

This code won’t deadlock.