1 files changed, 81 insertions, 17 deletions

diff --git a/include/linux/pid.h b/include/linux/pid.h
index 5b2fcb1..29960b0 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -1,55 +1,119 @@
 #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 */