ptrace_check_attach: remove unneeded ->signal != NULL check

It is not possible to see the PT_PTRACED task without ->signal/sighand under
tasklist_lock, release_task() does ptrace_unlink() first.  If the task was
already released before, ptrace_attach() can't succeed and set PT_PTRACED.
Remove this check.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Roland McGrath <roland@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/ptrace.c

@@ -99,9 +99,12 @@ int ptrace_check_attach(struct task_struct *child, int kill)
 	 * be changed by us so it's not changing right after this.
 	 */
 	read_lock(&tasklist_lock);
-	if ((child->ptrace & PT_PTRACED) && child->parent == current &&
-	     child->signal != NULL) {
+	if ((child->ptrace & PT_PTRACED) && child->parent == current) {
 		ret = 0;
+		/*
+		 * child->sighand can't be NULL, release_task()
+		 * does ptrace_unlink() before __exit_signal().
+		 */
 		spin_lock_irq(&child->sighand->siglock);
 		if (task_is_stopped(child))
 			child->state = TASK_TRACED;