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
  44 /*
  45  * struct upid is used to get the id of the struct pid, as it is
  46  * seen in particular namespace. Later the struct pid is found with
  47  * find_pid_ns() using the int nr and struct pid_namespace *ns.
  48  */
  49
  50 struct upid {
  51         /* Try to keep pid_chain in the same cacheline as nr for find_pid */
  52         int nr;
  53         struct pid_namespace *ns;
  54         struct hlist_node pid_chain;
  55 };
  56
  57 struct pid
  58 {
  59         atomic_t count;
  60         /* Try to keep pid_chain in the same cacheline as nr for find_pid */
  61         int nr;
  62         struct hlist_node pid_chain;
  63         /* lists of tasks that use this pid */
  64         struct hlist_head tasks[PIDTYPE_MAX];
  65         struct rcu_head rcu;
  66         int level;
  67         struct upid numbers[1];
  68 };
  69
  70 extern struct pid init_struct_pid;
  71
  72 struct pid_link
  73 {
  74         struct hlist_node node;
  75         struct pid *pid;
  76 };
  77
  78 static inline struct pid *get_pid(struct pid *pid)
  79 {
  80         if (pid)
  81                 atomic_inc(&pid->count);
  82         return pid;
  83 }
  84
  85 extern void FASTCALL(put_pid(struct pid *pid));
  86 extern struct task_struct *FASTCALL(pid_task(struct pid *pid, enum pid_type));
  87 extern struct task_struct *FASTCALL(get_pid_task(struct pid *pid,
  88                                                 enum pid_type));
  89
  90 extern struct pid *get_task_pid(struct task_struct *task, enum pid_type type);
  91
  92 /*
  93  * attach_pid() and detach_pid() must be called with the tasklist_lock
  94  * write-held.
  95  */
  96 extern int FASTCALL(attach_pid(struct task_struct *task,
  97                                 enum pid_type type, struct pid *pid));
  98 extern void FASTCALL(detach_pid(struct task_struct *task, enum pid_type));
  99 extern void FASTCALL(transfer_pid(struct task_struct *old,
 100                                   struct task_struct *new, enum pid_type));
 101
 102 /*
 103  * look up a PID in the hash table. Must be called with the tasklist_lock
 104  * or rcu_read_lock() held.
 105  */
 106 extern struct pid *FASTCALL(find_pid(int nr));
 107
 108 /*
 109  * Lookup a PID in the hash table, and return with it's count elevated.
 110  */
 111 extern struct pid *find_get_pid(int nr);
 112 extern struct pid *find_ge_pid(int nr);
 113
 114 extern struct pid *alloc_pid(void);
 115 extern void FASTCALL(free_pid(struct pid *pid));
 116
 117 static inline pid_t pid_nr(struct pid *pid)
 118 {
 119         pid_t nr = 0;
 120         if (pid)
 121                 nr = pid->nr;
 122         return nr;
 123 }
 124
 125 #define do_each_pid_task(pid, type, task)                               \
 126         do {                                                            \
 127                 struct hlist_node *pos___;                              \
 128                 if (pid != NULL)                                        \
 129                         hlist_for_each_entry_rcu((task), pos___,        \
 130                                 &pid->tasks[type], pids[type].node) {
 131
 132 #define while_each_pid_task(pid, type, task)                            \
 133                         }                                               \
 134         } while (0)
 135
 136 #endif /* _LINUX_PID_H */