bpf: task_group_seq_get_next: cleanup the usage of get/put_task_struct

get_pid_task() makes no sense, the code does put_task_struct() soon after.
Use find_task_by_pid_ns() instead of find_pid_ns + get_pid_task and kill
put_task_struct(), this allows to do get_task_struct() only once before
return.

While at it, kill the unnecessary "if (!pid)" check in the "if (!*tid)"
block, this matches the next usage of find_pid_ns() + get_pid_task() in
this function.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230905154649.GA24935@redhat.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/task_iter.c

@@ -42,9 +42,6 @@ static struct task_struct *task_group_seq_get_next(struct bpf_iter_seq_task_comm
 	if (!*tid) {
 		/* The first time, the iterator calls this function. */
 		pid = find_pid_ns(common->pid, common->ns);
-		if (!pid)
-			return NULL;
-
 		task = get_pid_task(pid, PIDTYPE_TGID);
 		if (!task)
 			return NULL;
@@ -66,17 +63,12 @@ static struct task_struct *task_group_seq_get_next(struct bpf_iter_seq_task_comm
 		return task;
 	}
 
-	pid = find_pid_ns(common->pid_visiting, common->ns);
-	if (!pid)
-		return NULL;
-
-	task = get_pid_task(pid, PIDTYPE_PID);
+	task = find_task_by_pid_ns(common->pid_visiting, common->ns);
 	if (!task)
 		return NULL;
 
 retry:
 	next_task = next_thread(task);
-	put_task_struct(task);
 
 	saved_tid = *tid;
 	*tid = __task_pid_nr_ns(next_task, PIDTYPE_PID, common->ns);
@@ -88,7 +80,6 @@ static struct task_struct *task_group_seq_get_next(struct bpf_iter_seq_task_comm
 		return NULL;
 	}
 
-	get_task_struct(next_task);
 	common->pid_visiting = *tid;
 
 	if (skip_if_dup_files && task->files == task->group_leader->files) {
@@ -96,6 +87,7 @@ static struct task_struct *task_group_seq_get_next(struct bpf_iter_seq_task_comm
 		goto retry;
 	}
 
+	get_task_struct(next_task);
 	return next_task;
 }