[SOLVED] looping over a process's parents, Linux kernel

Issue

The following code loops over the ancestors of curr_task, all the way to the swapper process (i.e the "most distant" ancestor of any Linux process), where it stops because swapper == swapper->parent.

while (curr_task != curr_task->parent)
       ...get curr_task info
       curr_task = curr_task->parent

The problem is I also want to get the swapper process's info. There are a couple obvious ways to do this (eg goto statement after the while loop, or a single if statement after the loop that gets the swapper info). However, these obvious solutions seems fairly inelegant, and so I'm looking for a more concise solution that doesn't involve the much-maligned goto or copy/pasting code.

Solution

Here's one way to do it: use another variable.

struct task_struct *t, *prev;
t = current;
do {
    // do something with t
    prev = t;
    t = t->parent;
} while (prev->pid != 0);

Or move that into a function:

int move_up(struct task **tp) {
    int was_swapper = ((*tp)->pid == 0);
    *tp = (*tp)->parent;
    return was_swapper;
}

// ...

struct task_struct *t = current;
do {
     // do something with t
} while (move_up(&t));

Answered By - Nate Eldredge

Answer Checked By - Candace Johnson (WPSolving Volunteer)