linux-kernel - Re: [PATCH] USB: serial: fix sysfs-attribute removal deadlock

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Message-ID: <Pine.LNX.4.44L0.1404250953090.1140-100000@iolanthe.rowland.org>
Date:	Fri, 25 Apr 2014 09:54:33 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Li Zhong <zhong@...ux.vnet.ibm.com>
cc:	Tejun Heo <tj@...nel.org>, Johan Hovold <jhovold@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<rafael.j.wysocki@...el.com>
Subject: Re: [PATCH] USB: serial: fix sysfs-attribute removal deadlock

On Fri, 25 Apr 2014, Li Zhong wrote:

> > I don't get why try_module_get() matters here.  We can't call into
> > ->store if the object at hand is already destroyed and the underlying
> > module can't go away if the target device is still alive.
> > try_module_get() doesn't actually protect the object.  Why does that
> > matter?  This is self removal, right?  Can you please take a look at
> > kernfs_remove_self()?
> 
> This is about one process writing something to driver attributes, and
> one process trying to unload this driver. 
> 
> I think try_module_get() could detect whether the driver is being
> unloaded, and if not, prevent it from being unloaded, so it could
> protect the object here by not allow the driver to be unloaded.

That isn't how try_module_get() works.  If the module is being 
unloaded, try_module_get() simply fails.  It does not prevent the 
module from being unloaded -- that's why its name begins with "try".

Alan Stern

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