Swift doesn’t only deal with dependencies, but with data conflicts as well. We can’t have two independent tasks working on the same data concurrently, even if the tasks have no dependencies between them. For this reason, each task locks the data it works on when it begins, and unlocks the data again once it’s finished. Data here refers to the particle (gas, gravity, stars,…) content of a cell as well as of the hierarchy of cells in the tree at levels closer to the root than the cell itself. By default, it is assumed that the task is doing work on hydro particles. If your task requires other particle types, you will need to specify that in src/task.c. Suppose you have implemented a task with type task_type_new that requires both stars and hydro particles:

/**
* @brief Try to lock the cells associated with this task.
*
*/

const enum task_types type = t->type;
const enum task_subtypes subtype = t->subtype;
struct cell *ci = t->ci, *cj = t->cj;

switch (type) {
/* lots of stuff */

/* is the data locked already? */
if (ci->hydro.hold || ci->stars.hold) return 0;

/* if it's not locked already, lock first particle type (hydro)
* if something locked it in the meantime, exit with failure. */
if (cell_locktree(ci) != 0) return 0;

/* if it's not locked already, lock first particle type (stars)
* if something locked it in the meantime, first unlock what you
* locked, and then exit with failure. */
if (cell_slocktree(ci) != 0) {
cell_unlocktree(ci);
return 0;
}

/* lots of other stuff */
}

/* lots of other stuff */

}


Similarly, don’t forget to write your unlocking routines in task_unlock() !