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Date:	Thu, 22 May 2014 10:26:57 +0100
From:	Dave Martin <>
To:	Greg Kroah-Hartman <>
Cc:	"" <>
Subject: Re: debugfs and module unloading

On Wed, May 21, 2014 at 11:01:14PM +0100, Greg Kroah-Hartman wrote:
> On Wed, May 21, 2014 at 05:48:28PM +0100, Dave P Martin wrote:
> > Hi there,
> > 
> > I have a quick debugfs question:
> > 
> > How can I protect against a module being unloaded while debugfs files it
> > provides are open?
> > 
> > I can do something like the following:


> > ... but it doesn't quite work.  debugfs_remove_recursive() prevents any
> > new file handles being opened, but files already open remain open -- so
> > it's still possible for the module text to get unloaded between the
> > module refcount being decreased inside module_put() and the time
> > my_debugfs_file_release() returns.
> > 
> > The scenario to consider is when a request to unload the module races
> > with closure of the last debugfs file.
> > 
> > 
> > The only obvious way I can see to solve this without changing the debugfs
> > code is to make the module impossible to unload by calling __module_get()
> > during initialisation, before any debugfs file is created.
> > 
> > 
> > A similar dependency problem exists when a pointer to some device
> > instance data is passed to debugfs_create_file().  For pluggable
> > devices, the device might go away at any time.  I hoped this could
> > be solved by calling get_device() ... put_device() in the debugfs file
> > open and release methods, but I found that a device instance can
> > get removed, and the module unloaded, even though the struct device
> > refcount is not zero.
> > 
> > 
> > Am I missing something?
> Nope, you are not, it's a known issue.  Al Viro is doing some core VFS
> work to help mitigate this, and I am working on converting debugfs to
> use kernfs which should also help resolve this problem.
> Don't unload modules automatically and you should be fine :)

OK, I will stop attempting half-baked workarounds of my own and keep
an eye out for changes in VFS.

Unloading the module certainly speeds up development work, but users
of it are unlikely to care...

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