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         /* lists of tasks that use this pid */
  61         struct hlist_head tasks[PIDTYPE_MAX];
  62         struct rcu_head rcu;
  63         int level;
  64         struct upid numbers[1];
  65 };
  66
  67 extern struct pid init_struct_pid;
  68
  69 struct pid_link
  70 {
  71         struct hlist_node node;
  72         struct pid *pid;
  73 };
  74
  75 static inline struct pid *get_pid(struct pid *pid)
  76 {
  77         if (pid)
  78                 atomic_inc(&pid->count);
  79         return pid;
  80 }
  81
  82 extern void put_pid(struct pid *pid);
  83 extern struct task_struct *pid_task(struct pid *pid, enum pid_type);
  84 extern struct task_struct *get_pid_task(struct pid *pid, enum pid_type);
  85
  86 extern struct pid *get_task_pid(struct task_struct *task, enum pid_type type);
  87
  88 /*
  89  * attach_pid() and detach_pid() must be called with the tasklist_lock
  90  * write-held.
  91  */
  92 extern int attach_pid(struct task_struct *task, enum pid_type type,
  93                       struct pid *pid);
  94 extern void detach_pid(struct task_struct *task, enum pid_type);
  95 extern void transfer_pid(struct task_struct *old, struct task_struct *new,
  96                          enum pid_type);
  97
  98 struct pid_namespace;
  99 extern struct pid_namespace init_pid_ns;
 100
 101 /*
 102  * look up a PID in the hash table. Must be called with the tasklist_lock
 103  * or rcu_read_lock() held.
 104  *
 105  * find_pid_ns() finds the pid in the namespace specified
 106  * find_pid() find the pid by its global id, i.e. in the init namespace
 107  * find_vpid() finr the pid by its virtual id, i.e. in the current namespace
 108  *
 109  * see also find_task_by_pid() set in include/linux/sched.h
 110  */
 111 extern struct pid *find_pid_ns(int nr, struct pid_namespace *ns);
 112 extern struct pid *find_vpid(int nr);
 113 extern struct pid *find_pid(int nr);
 114
 115 /*
 116  * Lookup a PID in the hash table, and return with it's count elevated.
 117  */
 118 extern struct pid *find_get_pid(int nr);
 119 extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
 120 int next_pidmap(struct pid_namespace *pid_ns, int last);
 121
 122 extern struct pid *alloc_pid(struct pid_namespace *ns);
 123 extern void free_pid(struct pid *pid);
 124
 125 /*
 126  * the helpers to get the pid's id seen from different namespaces
 127  *
 128  * pid_nr()    : global id, i.e. the id seen from the init namespace;
 129  * pid_vnr()   : virtual id, i.e. the id seen from the pid namespace of
 130  *               current.
 131  * pid_nr_ns() : id seen from the ns specified.
 132  *
 133  * see also task_xid_nr() etc in include/linux/sched.h
 134  */
 135
 136 static inline pid_t pid_nr(struct pid *pid)
 137 {
 138         pid_t nr = 0;
 139         if (pid)
 140                 nr = pid->numbers[0].nr;
 141         return nr;
 142 }
 143
 144 pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns);
 145 pid_t pid_vnr(struct pid *pid);
 146
 147 #define do_each_pid_task(pid, type, task)                               \
 148         do {                                                            \
 149                 struct hlist_node *pos___;                              \
 150                 if (pid != NULL)                                        \
 151                         hlist_for_each_entry_rcu((task), pos___,        \
 152                                 &pid->tasks[type], pids[type].node) {
 153
 154                         /*
 155                          * Both old and new leaders may be attached to
 156                          * the same pid in the middle of de_thread().
 157                          */
 158 #define while_each_pid_task(pid, type, task)                            \
 159                                 if (type == PIDTYPE_PID)                \
 160                                         break;                          \
 161                         }                                               \
 162         } while (0)
 163
 164 #endif /* _LINUX_PID_H */