ptrace_get_task_struct: s/tasklist/rcu/, make it static

- Use rcu_read_lock() instead of tasklist_lock to find/get the task
  in ptrace_get_task_struct().

- Make it static, it has no callers outside of ptrace.c.

- The comment doesn't match the reality, this helper does not do
  any checks. Beacuse it is really trivial and static I removed the
  whole comment.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/ptrace.h

@@ -81,7 +81,6 @@
 
 
 extern long arch_ptrace(struct task_struct *child, long request, long addr, long data);
-extern struct task_struct *ptrace_get_task_struct(pid_t pid);
 extern int ptrace_traceme(void);
 extern int ptrace_readdata(struct task_struct *tsk, unsigned long src, char __user *dst, int len);
 extern int ptrace_writedata(struct task_struct *tsk, char __user *src, unsigned long dst, int len);

kernel/ptrace.c

@@ -581,26 +581,16 @@ int ptrace_request(struct task_struct *child, long request,
 	return ret;
 }
 
-/**
- * ptrace_get_task_struct  --  grab a task struct reference for ptrace
- * @pid:       process id to grab a task_struct reference of
- *
- * This function is a helper for ptrace implementations.  It checks
- * permissions and then grabs a task struct for use of the actual
- * ptrace implementation.
- *
- * Returns the task_struct for @pid or an ERR_PTR() on failure.
- */
-struct task_struct *ptrace_get_task_struct(pid_t pid)
+static struct task_struct *ptrace_get_task_struct(pid_t pid)
 {
 	struct task_struct *child;
 
-	read_lock(&tasklist_lock);
+	rcu_read_lock();
 	child = find_task_by_vpid(pid);
 	if (child)
 		get_task_struct(child);
+	rcu_read_unlock();
 
-	read_unlock(&tasklist_lock);
 	if (!child)
 		return ERR_PTR(-ESRCH);
 	return child;