include/linux/pid.h

   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 */