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

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Message-ID: <CAA93t1oENvBNDUYBNk2FKjwUSt77y-amLgV9h-fu7TPpiPJ98A@mail.gmail.com>
Date:	Mon, 28 Dec 2015 12:51:32 -0800
From:	Rajat Jain <rajatxjain@...il.com>
To:	Greg Kroah-Hartman <greg@...ah.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

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.

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?

Would hanging those files under the sysfs node for the device sound
any better (by representing each "context" using an embedded kobject)?

Thanks,

Rajat

On Mon, Dec 28, 2015 at 12:31 PM, Greg Kroah-Hartman <greg@...ah.com> wrote:
> On Mon, Dec 28, 2015 at 12:27:22PM -0800, Rajat Jain wrote:
>> Hi,
>>
>> I wanted to understand the behavior taken when a module calls
>> debugfs_remove_recursive() on a directory, while files under that
>> directory may still be in use by the userspace (for instance an
>> ongoing read / write operation).
>
> Bad things :(
>
>> Does the function wait
>>
>> (1) until all the currently executing file operation methods
>> (read/write/map etc) have returned?
>
> Nope.
>
>> OR
>> (2) until the user has given up all references (descriptors) to the
>> files under the directory (i.e. until release() method has been
>> called)?
>
> Nope.
>
> There are some patches on the mailing list that I need to review that
> hopefully should resolve this problem, as it's been known for a very
> long time.
>
> In short, just don't remove debugfs files unless your module is
> unloading, and all should be good as modules are never auto-unloaded.
> If you remove debugfs files when a device is removed, be careful.
>
> thanks,
>
> greg k-h
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