linux-kernel - Re: debugfs_remove_recursive() while a file is in use by userspace

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Message-ID: <20151228211753.GB24118@kroah.com>
Date:	Mon, 28 Dec 2015 13:17:53 -0800
From:	Greg Kroah-Hartman <greg@...ah.com>
To:	Rajat Jain <rajatxjain@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-newbie@...r.kernel.org
Subject: Re: debugfs_remove_recursive() while a file is in use by userspace

On Mon, Dec 28, 2015 at 01:11:53PM -0800, Rajat Jain wrote:
> On Mon, Dec 28, 2015 at 12:58 PM, Greg Kroah-Hartman <greg@...ah.com> wrote:
> > On Mon, Dec 28, 2015 at 12:51:32PM -0800, Rajat Jain wrote:
> >> Thanks Greg and Al for the quick turnaround.
> >>
> >> Essentially I have a device that supports something called "contexts"
> >> that can be "created" and "destroyed" during the life of the device. I
> >> want to expose some debug files for the context when it is created,
> >> and destroy the files when the context is destroyed. However, I'm not
> >> sure how do I ensure that the user is not in the middle of reading /
> >> writing / mmaping to those files. Also how do I know that user is
> >> still not holding a reference to the file structure.
> >
> > You don't.
> >
> >> It seems like debugfs is currently not a good choice for this? Would
> >> you recommend me to any other fs or subsystem that I should use for
> >> this?
> >
> > What exactly do you need to export to userspace and for what purpose?
> > For debugging-only stuff, sure, use debugfs, but don't rely on it for
> > any "real" tools, only your own debugging.
> 
> I'm actually writing a driver that would expose a "dummy device" to a
> real driver. The dummy driver relies on user space to feed in the
> device attributes (no of supported contexts etc).

That sounds "odd".

Why not configfs?

> I am now thinking that a character device interface to user space may
> actually be a better choice.

Not really, you would now have to parse things in a character driver
write() callback, not very good or robust.

> Question: Does cdev_del() ensure that all references to the file are
> dropped before it returns?

No, but when the reference is dropped, everything cleans up properly.

thanks,

greg k-h
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