Why do we need list_for_each_safe() in for deleting nodes in kernel linked list?

Question

I\'m learning how to use the kernel linked-list API from list.h.

I learned that I need to use list_for_each_safe() when deleting nodes off with list_d

Answer 1

pos = start;
del(pos);
pos = pos->next;

as opposed to

pos = start;
n = pos->next;
del(pos);
pos = n;

if del() is free() and memset(), pos->next is undefined

Answer 2

That is necessary because list_del internally modifies the value of pos fields. In your example the loop body even frees the memory occupied by pos. Suppose that you would use unsafe version of the loop:

for (pos = (head)->next; pos != (head); pos = pos->next)

After executing the loop body pos pointer becomes invalid breaking the increment expression: pos = pos->next.

As opposite, the safe foreach pre-saves the value of pos->next in a temporary variable and then refers to the latter instead of dereferencing pos:

for (pos = (head)->next, n = pos->next; pos != (head); \
    pos = n, n = pos->next)