1 files changed, 81 insertions, 15 deletions

diff --git a/include/linux/pid.h b/include/linux/pid.h
index 5b9082cc600f..29960b03bef7 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -1,6 +1,8 @@
 #ifndef _LINUX_PID_H
 #define _LINUX_PID_H
 
+#include <linux/rcupdate.h>
+
 enum pid_type
 {
         PIDTYPE_PID,
@@ -9,45 +11,109 @@ enum pid_type
         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 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 */