3.2.2.4. The process list

The first example of a doubly linked list we will examine is the process list, a list that links together all existing process descriptors. Each task_struct structure includes a tasks field of type list_head whose prev and next fields point, respectively, to the previous and to the next task_struct element.

The head of the process list is the init_task task_struct descriptor; it is the process descriptor of the so-called process 0 or swapper (see the section "Kernel Threads" later in this chapter). The tasks->prev field of init_task points to the tasks field of the process descriptor inserted last in the list.

The SET_LINKS and REMOVE_LINKS macros are used to insert and to remove a process descriptor in the process list, respectively. These macros also take care of the parenthood relationship of the process (see the section "How Processes Are Organized" later in this chapter).

Another useful macro, called for_each_process , scans the whole process list. It is defined as:

#define for_each_process(p) \
for (p=&init_task; (p=list_entry((p)->tasks.next, \
struct task_struct, tasks) \
) != &init_task; )

The macro is the loop control statement after which the kernel programmer supplies the loop. Notice how the init_task process descriptor just plays the role of list header. The macro starts by moving past init_task to the next task and continues until it reaches init_task again (thanks to the circularity of the list). At each iteration, the variable passed as the argument of the macro contains the address of the currently scanned process descriptor, as returned by the list_entry macro.

NOTE: 在multiprocessor中，是否是每个processor都有一个process list，还是说所有的process descriptor都放在一个process list中？