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 struct pid_link 55 { 56 struct hlist_node node; 57 struct pid *pid; 58 }; 59 60 static inline struct pid *get_pid(struct pid *pid) 61 { 62 if (pid) 63 atomic_inc(&pid->count); 64 return pid; 65 } 66 67 extern void FASTCALL(put_pid(struct pid *pid)); 68 extern struct task_struct *FASTCALL(pid_task(struct pid *pid, enum pid_type)); 69 extern struct task_struct *FASTCALL(get_pid_task(struct pid *pid, 70 enum pid_type)); 71 72 extern struct pid *get_task_pid(struct task_struct *task, enum pid_type type); 73 74 /* 75 * attach_pid() and detach_pid() must be called with the tasklist_lock 76 * write-held. 77 */ 78 extern int FASTCALL(attach_pid(struct task_struct *task, 79 enum pid_type type, int nr)); 80 81 extern void FASTCALL(detach_pid(struct task_struct *task, enum pid_type)); 82 extern void FASTCALL(transfer_pid(struct task_struct *old, 83 struct task_struct *new, enum pid_type)); 84 85 /* 86 * look up a PID in the hash table. Must be called with the tasklist_lock 87 * or rcu_read_lock() held. 88 */ 89 extern struct pid *FASTCALL(find_pid(int nr)); 90 91 /* 92 * Lookup a PID in the hash table, and return with it's count elevated. 93 */ 94 extern struct pid *find_get_pid(int nr); 95 extern struct pid *find_ge_pid(int nr); 96 97 extern struct pid *alloc_pid(void); 98 extern void FASTCALL(free_pid(struct pid *pid)); 99 100 static inline pid_t pid_nr(struct pid *pid) 101 { 102 pid_t nr = 0; 103 if (pid) 104 nr = pid->nr; 105 return nr; 106 } 107 108 #define do_each_pid_task(pid, type, task) \ 109 do { \ 110 struct hlist_node *pos___; \ 111 if (pid != NULL) \ 112 hlist_for_each_entry_rcu((task), pos___, \ 113 &pid->tasks[type], pids[type].node) { 114 115 #define while_each_pid_task(pid, type, task) \ 116 } \ 117 } while (0) 118 119 #endif /* _LINUX_PID_H */ 120