syscall_regfunc() && TIF_SYSCALL_TRACEPOINT

[Oleg Nesterov <oleg@redhat.com>]

Hello.

I've looked at syscall_regfunc/unregfunc by accident, and I am
a bit confused...

	void syscall_regfunc(void)
	{
		unsigned long flags;
		struct task_struct *g, *t;

		if (!sys_tracepoint_refcount) {
			read_lock_irqsave(&tasklist_lock, flags);

Why _irqsave? write_lock(tasklist) needs to disable irqs, but read_
doesn't. Any subtle reason I missed?

			do_each_thread(g, t) {
				/* Skip kernel threads. */
				if (t->mm)

We should check PF_KTHREAD, not ->mm.

					set_tsk_thread_flag(t, TIF_SYSCALL_TRACEPOINT);

But the main question is, can't we race with clone() and miss the
new child? The new task is not "visible" to do_each_thread() until
copy_process()->list_add_tail_rcu(thread_group/init_task.tasks).

Don't we need something like the patch below?

Oleg.


--- x/kernel/fork.c
+++ x/kernel/fork.c
@@ -1446,7 +1446,12 @@ static struct task_struct *copy_process(
 
 	total_forks++;
 	spin_unlock(&current->sighand->siglock);
+#ifdef CONFIG_TRACEPOINTS
+	if (unlikely(test_thread_flag(TIF_SYSCALL_TRACEPOINT)))
+		set_tsk_thread_flag(p, TIF_SYSCALL_TRACEPOINT);
+#endif
 	write_unlock_irq(&tasklist_lock);
+
 	proc_fork_connector(p);
 	cgroup_post_fork(p);
 	if (clone_flags & CLONE_THREAD)