Linux kernel mirror (for testing)
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linux
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
39 * pid value is reused we don't mistakenly refer to new processes.
40 */
41
42struct pid
43{
44 atomic_t count;
45 /* Try to keep pid_chain in the same cacheline as nr for find_pid */
46 int nr;
47 struct hlist_node pid_chain;
48 /* lists of tasks that use this pid */
49 struct hlist_head tasks[PIDTYPE_MAX];
50 struct rcu_head rcu;
51};
52
53struct pid_link
54{
55 struct hlist_node node;
56 struct pid *pid;
57};
58
59static inline struct pid *get_pid(struct pid *pid)
60{
61 if (pid)
62 atomic_inc(&pid->count);
63 return pid;
64}
65
66extern void FASTCALL(put_pid(struct pid *pid));
67extern struct task_struct *FASTCALL(pid_task(struct pid *pid, enum pid_type));
68extern struct task_struct *FASTCALL(get_pid_task(struct pid *pid,
69 enum pid_type));
70
71/*
72 * attach_pid() and detach_pid() must be called with the tasklist_lock
73 * write-held.
74 */
75extern int FASTCALL(attach_pid(struct task_struct *task,
76 enum pid_type type, int nr));
77
78extern void FASTCALL(detach_pid(struct task_struct *task, enum pid_type));
79
80/*
81 * look up a PID in the hash table. Must be called with the tasklist_lock
82 * or rcu_read_lock() held.
83 */
84extern struct pid *FASTCALL(find_pid(int nr));
85
86/*
87 * Lookup a PID in the hash table, and return with it's count elevated.
88 */
89extern struct pid *find_get_pid(int nr);
90
91extern struct pid *alloc_pid(void);
92extern void FASTCALL(free_pid(struct pid *pid));
93
94#define pid_next(task, type) \
95 ((task)->pids[(type)].node.next)
96
97#define pid_next_task(task, type) \
98 hlist_entry(pid_next(task, type), struct task_struct, \
99 pids[(type)].node)
100
101
102/* We could use hlist_for_each_entry_rcu here but it takes more arguments
103 * than the do_each_task_pid/while_each_task_pid. So we roll our own
104 * to preserve the existing interface.
105 */
106#define do_each_task_pid(who, type, task) \
107 if ((task = find_task_by_pid_type(type, who))) { \
108 prefetch(pid_next(task, type)); \
109 do {
110
111#define while_each_task_pid(who, type, task) \
112 } while (pid_next(task, type) && ({ \
113 task = pid_next_task(task, type); \
114 rcu_dereference(task); \
115 prefetch(pid_next(task, type)); \
116 1; }) ); \
117 }
118
119#endif /* _LINUX_PID_H */