1 #ifndef _LINUX_PID_H 2 #define _LINUX_PID_H 3 4 #include <linux/rcupdate.h> 5 6 enum 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 43 struct 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 54 extern struct pid init_struct_pid; 55 56 struct pid_link 57 { 58 struct hlist_node node; 59 struct pid *pid; 60 }; 61 62 static inline struct pid *get_pid(struct pid *pid) 63 { 64 if (pid) 65 atomic_inc(&pid->count); 66 return pid; 67 } 68 69 extern void FASTCALL(put_pid(struct pid *pid)); 70 extern struct task_struct *FASTCALL(pid_task(struct pid *pid, enum pid_type)); 71 extern struct task_struct *FASTCALL(get_pid_task(struct pid *pid, 72 enum pid_type)); 73 74 extern 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 */ 80 extern int FASTCALL(attach_pid(struct task_struct *task, 81 enum pid_type type, struct pid *pid)); 82 extern void FASTCALL(detach_pid(struct task_struct *task, enum pid_type)); 83 extern 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 */ 90 extern 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 */ 95 extern struct pid *find_get_pid(int nr); 96 extern struct pid *find_ge_pid(int nr); 97 98 extern struct pid *alloc_pid(void); 99 extern void FASTCALL(free_pid(struct pid *pid)); 100 101 static 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