#ifndef _LINUX_PID_H
#define _LINUX_PID_H
+#include <linux/rcupdate.h>
+
enum pid_type
{
PIDTYPE_PID,
- PIDTYPE_TGID,
PIDTYPE_PGID,
PIDTYPE_SID,
PIDTYPE_MAX
};
+/*
+ * What is struct pid?
+ *
+ * A struct pid is the kernel's internal notion of a process identifier.
+ * It refers to individual tasks, process groups, and sessions. While
+ * there are processes attached to it the struct pid lives in a hash
+ * table, so it and then the processes that it refers to can be found
+ * quickly from the numeric pid value. The attached processes may be
+ * quickly accessed by following pointers from struct pid.
+ *
+ * Storing pid_t values in the kernel and refering to them later has a
+ * problem. The process originally with that pid may have exited and the
+ * pid allocator wrapped, and another process could have come along
+ * and been assigned that pid.
+ *
+ * Referring to user space processes by holding a reference to struct
+ * task_struct has a problem. When the user space process exits
+ * the now useless task_struct is still kept. A task_struct plus a
+ * stack consumes around 10K of low kernel memory. More precisely
+ * this is THREAD_SIZE + sizeof(struct task_struct). By comparison
+ * a struct pid is about 64 bytes.
+ *
+ * Holding a reference to struct pid solves both of these problems.
+ * It is small so holding a reference does not consume a lot of
+ * resources, and since a new struct pid is allocated when the numeric
+ * pid value is reused we don't mistakenly refer to new processes.
+ */
+
struct pid
{
+ atomic_t count;
/* Try to keep pid_chain in the same cacheline as nr for find_pid */
int nr;
struct hlist_node pid_chain;
- /* list of pids with the same nr, only one of them is in the hash */
- struct list_head pid_list;
+ /* lists of tasks that use this pid */
+ struct hlist_head tasks[PIDTYPE_MAX];
+ struct rcu_head rcu;
};
-#define pid_task(elem, type) \
- list_entry(elem, struct task_struct, pids[type].pid_list)
+struct pid_link
+{
+ struct hlist_node node;
+ struct pid *pid;
+};
+
+static inline struct pid *get_pid(struct pid *pid)
+{
+ if (pid)
+ atomic_inc(&pid->count);
+ return pid;
+}
+
+extern void FASTCALL(put_pid(struct pid *pid));
+extern struct task_struct *FASTCALL(pid_task(struct pid *pid, enum pid_type));
+extern struct task_struct *FASTCALL(get_pid_task(struct pid *pid,
+ enum pid_type));
/*
* attach_pid() and detach_pid() must be called with the tasklist_lock
* write-held.
*/
-extern int FASTCALL(attach_pid(struct task_struct *task, enum pid_type type, int nr));
+extern int FASTCALL(attach_pid(struct task_struct *task,
+ enum pid_type type, int nr));
extern void FASTCALL(detach_pid(struct task_struct *task, enum pid_type));
/*
* look up a PID in the hash table. Must be called with the tasklist_lock
- * held.
+ * or rcu_read_lock() held.
+ */
+extern struct pid *FASTCALL(find_pid(int nr));
+
+/*
+ * Lookup a PID in the hash table, and return with it's count elevated.
*/
-extern struct pid *FASTCALL(find_pid(enum pid_type, int));
+extern struct pid *find_get_pid(int nr);
-extern int alloc_pidmap(void);
-extern void FASTCALL(free_pidmap(int));
-extern void switch_exec_pids(struct task_struct *leader, struct task_struct *thread);
+extern struct pid *alloc_pid(void);
+extern void FASTCALL(free_pid(struct pid *pid));
+#define pid_next(task, type) \
+ ((task)->pids[(type)].node.next)
+
+#define pid_next_task(task, type) \
+ hlist_entry(pid_next(task, type), struct task_struct, \
+ pids[(type)].node)
+
+
+/* We could use hlist_for_each_entry_rcu here but it takes more arguments
+ * than the do_each_task_pid/while_each_task_pid. So we roll our own
+ * to preserve the existing interface.
+ */
#define do_each_task_pid(who, type, task) \
if ((task = find_task_by_pid_type(type, who))) { \
- prefetch((task)->pids[type].pid_list.next); \
+ prefetch(pid_next(task, type)); \
do {
#define while_each_task_pid(who, type, task) \
- } while (task = pid_task((task)->pids[type].pid_list.next,\
- type), \
- prefetch((task)->pids[type].pid_list.next), \
- hlist_unhashed(&(task)->pids[type].pid_chain)); \
- } \
+ } while (pid_next(task, type) && ({ \
+ task = pid_next_task(task, type); \
+ rcu_dereference(task); \
+ prefetch(pid_next(task, type)); \
+ 1; }) ); \
+ }
#endif /* _LINUX_PID_H */
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