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 39 * pid value is reused we don't mistakenly refer to new processes. 40 */ 41 42 struct 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 53 struct pid_link 54 { 55 struct hlist_node node; 56 struct pid *pid; 57 }; 58 59 static inline struct pid *get_pid(struct pid *pid) 60 { 61 if (pid) 62 atomic_inc(&pid->count); 63 return pid; 64 } 65 66 extern void FASTCALL(put_pid(struct pid *pid)); 67 extern struct task_struct *FASTCALL(pid_task(struct pid *pid, enum pid_type)); 68 extern 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 */ 75 extern int FASTCALL(attach_pid(struct task_struct *task, 76 enum pid_type type, int nr)); 77 78 extern 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 */ 84 extern 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 */ 89 extern struct pid *find_get_pid(int nr); 90 91 extern struct pid *alloc_pid(void); 92 extern 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 */ 120