1#ifndef _LINUX_PID_H
2#define _LINUX_PID_H
3
4#include <linux/rcupdate.h>
5
6enum pid_type
7{
8	PIDTYPE_PID,
9	PIDTYPE_PGID,
10	PIDTYPE_SID,
11	PIDTYPE_MAX
12};
13
14/*
15 * What is struct pid?
16 *
17 * A struct pid is the kernel's internal notion of a process identifier.
18 * It refers to individual tasks, process groups, and sessions.  While
19 * there are processes attached to it the struct pid lives in a hash
20 * table, so it and then the processes that it refers to can be found
21 * quickly from the numeric pid value.  The attached processes may be
22 * quickly accessed by following pointers from struct pid.
23 *
24 * Storing pid_t values in the kernel and refering to them later has a
25 * problem.  The process originally with that pid may have exited and the
26 * pid allocator wrapped, and another process could have come along
27 * and been assigned that pid.
28 *
29 * Referring to user space processes by holding a reference to struct
30 * task_struct has a problem.  When the user space process exits
31 * the now useless task_struct is still kept.  A task_struct plus a
32 * stack consumes around 10K of low kernel memory.  More precisely
33 * this is THREAD_SIZE + sizeof(struct task_struct).  By comparison
34 * a struct pid is about 64 bytes.
35 *
36 * Holding a reference to struct pid solves both of these problems.
37 * It is small so holding a reference does not consume a lot of
38 * resources, and since a new struct pid is allocated when the numeric pid
39 * value is reused (when pids wrap around) we don't mistakenly refer to new
40 * processes.
41 */
42
43struct pid
44{
45	atomic_t count;
46	/* Try to keep pid_chain in the same cacheline as nr for find_pid */
47	int nr;
48	struct hlist_node pid_chain;
49	/* lists of tasks that use this pid */
50	struct hlist_head tasks[PIDTYPE_MAX];
51	struct rcu_head rcu;
52};
53
54extern struct pid init_struct_pid;
55
56struct pid_link
57{
58	struct hlist_node node;
59	struct pid *pid;
60};
61
62static inline struct pid *get_pid(struct pid *pid)
63{
64	if (pid)
65		atomic_inc(&pid->count);
66	return pid;
67}
68
69extern void FASTCALL(put_pid(struct pid *pid));
70extern struct task_struct *FASTCALL(pid_task(struct pid *pid, enum pid_type));
71extern struct task_struct *FASTCALL(get_pid_task(struct pid *pid,
72						enum pid_type));
73
74extern struct pid *get_task_pid(struct task_struct *task, enum pid_type type);
75
76/*
77 * attach_pid() and detach_pid() must be called with the tasklist_lock
78 * write-held.
79 */
80extern int FASTCALL(attach_pid(struct task_struct *task,
81				enum pid_type type, struct pid *pid));
82extern void FASTCALL(detach_pid(struct task_struct *task, enum pid_type));
83extern void FASTCALL(transfer_pid(struct task_struct *old,
84				  struct task_struct *new, enum pid_type));
85
86/*
87 * look up a PID in the hash table. Must be called with the tasklist_lock
88 * or rcu_read_lock() held.
89 */
90extern struct pid *FASTCALL(find_pid(int nr));
91
92/*
93 * Lookup a PID in the hash table, and return with it's count elevated.
94 */
95extern struct pid *find_get_pid(int nr);
96extern struct pid *find_ge_pid(int nr);
97
98extern struct pid *alloc_pid(void);
99extern void FASTCALL(free_pid(struct pid *pid));
100
101static inline pid_t pid_nr(struct pid *pid)
102{
103	pid_t nr = 0;
104	if (pid)
105		nr = pid->nr;
106	return nr;
107}
108
109#define do_each_pid_task(pid, type, task)				\
110	do {								\
111		struct hlist_node *pos___;				\
112		if (pid != NULL)					\
113			hlist_for_each_entry_rcu((task), pos___,	\
114				&pid->tasks[type], pids[type].node) {
115
116#define while_each_pid_task(pid, type, task)				\
117			}						\
118	} while (0)
119
120#endif /* _LINUX_PID_H */
121