---

yaml
---
r: 24929
b: refs/heads/master
c: 92476d7
h: refs/heads/master
i:
  24927: cb18bfd
v: v3

[refs]

@@ -1,2 +1,2 @@
 ---
-refs/heads/master: 8c7904a00b06d2ee51149794b619e07369fcf9d4
+refs/heads/master: 92476d7fc0326a409ab1d3864a04093a6be9aca7

trunk/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 */

trunk/include/linux/sched.h

@@ -760,7 +760,7 @@ struct task_struct {
 	struct task_struct *group_leader;	/* threadgroup leader */
 
 	/* PID/PID hash table linkage. */
-	struct pid pids[PIDTYPE_MAX];
+	struct pid_link pids[PIDTYPE_MAX];
 	struct list_head thread_group;
 
 	struct completion *vfork_done;		/* for vfork() */
@@ -899,7 +899,7 @@ static inline pid_t process_group(struct task_struct *tsk)
  */
 static inline int pid_alive(struct task_struct *p)
 {
-	return p->pids[PIDTYPE_PID].nr != 0;
+	return p->pids[PIDTYPE_PID].pid != NULL;
 }
 
 extern void free_task(struct task_struct *tsk);

trunk/kernel/fork.c

@@ -1315,17 +1315,19 @@ long do_fork(unsigned long clone_flags,
 {
 	struct task_struct *p;
 	int trace = 0;
-	long pid = alloc_pidmap();
+	struct pid *pid = alloc_pid();
+	long nr;
 
-	if (pid < 0)
+	if (!pid)
 		return -EAGAIN;
+	nr = pid->nr;
 	if (unlikely(current->ptrace)) {
 		trace = fork_traceflag (clone_flags);
 		if (trace)
 			clone_flags |= CLONE_PTRACE;
 	}
 
-	p = copy_process(clone_flags, stack_start, regs, stack_size, parent_tidptr, child_tidptr, pid);
+	p = copy_process(clone_flags, stack_start, regs, stack_size, parent_tidptr, child_tidptr, nr);
 	/*
 	 * Do this prior waking up the new thread - the thread pointer
 	 * might get invalid after that point, if the thread exits quickly.
@@ -1352,7 +1354,7 @@ long do_fork(unsigned long clone_flags,
 			p->state = TASK_STOPPED;
 
 		if (unlikely (trace)) {
-			current->ptrace_message = pid;
+			current->ptrace_message = nr;
 			ptrace_notify ((trace << 8) | SIGTRAP);
 		}
 
@@ -1362,10 +1364,10 @@ long do_fork(unsigned long clone_flags,
 				ptrace_notify ((PTRACE_EVENT_VFORK_DONE << 8) | SIGTRAP);
 		}
 	} else {
-		free_pidmap(pid);
-		pid = PTR_ERR(p);
+		free_pid(pid);
+		nr = PTR_ERR(p);
 	}
-	return pid;
+	return nr;
 }
 
 #ifndef ARCH_MIN_MMSTRUCT_ALIGN