llist: Clarify comments about when locking is needed

/*

 * Lock-less NULL terminated single linked list

 *

 * If there are multiple producers and multiple consumers, llist_add

 * can be used in producers and llist_del_all can be used in

 * consumers.  They can work simultaneously without lock.  But

 * llist_del_first can not be used here.  Because llist_del_first

 * depends on list->first->next does not changed if list->first is not

 * changed during its operation, but llist_del_first, llist_add,

 * llist_add (or llist_del_all, llist_add, llist_add) sequence in

 * another consumer may violate that.

 *

 * If there are multiple producers and one consumer, llist_add can be

 * used in producers and llist_del_all or llist_del_first can be used

 * in the consumer.

 *

 * This can be summarized as follow:

 * Cases where locking is not needed:

 * If there are multiple producers and multiple consumers, llist_add can be

 * used in producers and llist_del_all can be used in consumers simultaneously

 * without locking. Also a single consumer can use llist_del_first while

 * multiple producers simultaneously use llist_add, without any locking.

 *

 * Cases where locking is needed:

 * If we have multiple consumers with llist_del_first used in one consumer, and

 * llist_del_first or llist_del_all used in other consumers, then a lock is

 * needed.  This is because llist_del_first depends on list->first->next not

 * changing, but without lock protection, there's no way to be sure about that

 * if a preemption happens in the middle of the delete operation and on being

 * preempted back, the list->first is the same as before causing the cmpxchg in

 * llist_del_first to succeed. For example, while a llist_del_first operation

 * is in progress in one consumer, then a llist_del_first, llist_add,

 * llist_add (or llist_del_all, llist_add, llist_add) sequence in another

 * consumer may cause violations.

 *

 * This can be summarized as follows:

 *

 *           |   add    | del_first |  del_all

 * add       |    -     |     -     |     -

 * del_first |          |     L     |     L

 * del_all   |          |           |     -

 *

 * Where "-" stands for no lock is needed, while "L" stands for lock

 * is needed.

 * Where, a particular row's operation can happen concurrently with a column's

 * operation, with "-" being no lock needed, while "L" being lock is needed.

 *

 * The list entries deleted via llist_del_all can be traversed with

 * traversing function such as llist_for_each etc.  But the list