Re: [PATCH] debugfs: fix potential infinite loop in debugfs_remove_recursive

[yukuai (C)]

On 2019/11/14 22:34, Steven Rostedt wrote:
> On Thu, 14 Nov 2019 14:59:04 +0800
> "yukuai (C)" <yukuai3@xxxxxxxxxx> wrote:
>
> > > Have you tried this patch with lockdep enabled and tried to hit this
> > > code path?
>
> > >    
> > You are right, I get the results with lockdep enabled:
>
> That was what I was afraid of :-(
>
> > [   64.314748] ============================================
> > [   64.315568] WARNING: possible recursive locking detected
> > [   64.316549] 5.4.0-rc7-dirty #5 Tainted: G           O
> > [   64.317398] --------------------------------------------
> > [   64.318230] rmmod/2607 is trying to acquire lock:
>
> > The warning will disappeare by adding
> > lockdep_set_novalidate_class(&child->d_lock) before calling
> > simple_empty(child). But I'm not sure It's the right modfication.
>
> I'm wondering if we should add a simple_empty_unlocked() that does
> simple_empty() without taking the lock, to allow us to call
> spin_lock_nested() on the child. Of course, I don't know how much
> nesting we allow as it calls the nesting too.
Do you think we can do this:
1. add a new enum type for dentry_d_lock_class:
enum dentry_d_lock_class
{
	DENTRY_D_LOCK_NORMAL, /* implicitly used by plain spin_lock() APIs. */
	DENTRY_D_LOCK_NESTED
	DENTRY_D_LOCK_NESTED_1 /* maybe another name */
};
2. use the new enum type in simple_empty
int simple_empty(struct dentry *dentry)
{
	spin_lock(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
	list_for_each_entry(child, &dentry->d_subdirs, d_child) {
		spin_lock_nested(&child->d_lock, DENTRY_D_LOCK_NESTED_1);
}

If you agree, I'll try to send a patch or patchset(with modification in 
debugfs_remove_recursive).

Thanks!
Yu Kuai