Re: debugfs: was: Re: [PATCH v4] printk: Userspace format enumeration support

[Chris Down]

Petr Mladek writes:
> > > > +	debugfs_remove(ps->file);
> > >
> > > IMHO, we should remove the file before we remove the way how
> > > to read it. This should be done in the opposite order
> > > than in store_printk_fmt_sec().
> >
> > There is a subtle issue with doing this as-is: debugfs_remove(ps->file)
> > cannot be called under printk_fmts_mutex, because we may deadlock due to a
> > pinned debugfs refcnt if debugfs_remove() and _show happen at the same time.
>
> Do we need to call debugfs_remove(ps->file) under printk_fmts_mutex?

Ah, my concern was simultaneous entries into remove_printk_fmt_sec (which would 
require setting a separate flag under the mutex), but now I think about it, the 
module notifier synchronously waits, so that can't happen anyway.

As such it should be safe to just do:

remove()
{
	mutex_lock(&printk_fmts_mutex);
	ps = find_printk_fmt_sec();
	mutex_unlock(&printk_fmts_mutex);

	if (!ps)
		return;

	/* waits for _show */
	debugfs_remove(ps->file);

	mutex_lock(&printk_fmts_mutex);
	/* Do the data structure teardown */
	mutex_unlock(&printk_fmts_mutex);
}

Sounds good to me, I'll do that for v5. Thanks! :-)