3.19. Task locking¶

DISPATCH uses OpenMP nested locks, with the API defined in omp/omp_locks_mod.f90.

A basic rule that needs to be respected with locks is to avoid that two objects of the same class are locked at the same time by a task – the situation below can clearly lead to a deadlock

thread 1:
  lock A(1)
    lock & unlock A(2)
  unlock A(1)
thread 2:
  lock A(2)
    lock & unlock A(1)
  unlock A(2)

On the other hand, if a thread locks one type of lock, and then a whole set of another type of locks inside the first one, this cannot lead to a deadlock, since either the outer locks are the same, and only one thread can do the set, or they are different, and they can negotiate. Hence locking the task list avoids any deadlock that could potentially be triggered if threads were not locked out.