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

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Date:	Fri, 25 Apr 2014 09:59:53 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Li Zhong <zhong@...ux.vnet.ibm.com>
cc:	Johan Hovold <jhovold@...il.com>, Tejun Heo <tj@...nel.org>,
	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:

> > No, this isn't self removal. The driver-attribute (not device-attribute)
> > store operation simply grabs a lock that is also held while the driver
> > is being deregistered at module unload. Taking a reference to the module
> > in this case will prevent deregistration while store is running.
> > 
> > But it seems like this can be solved for usb-serial by simply not
> > holding the lock while deregistering.
> 
> I didn't look carefully about this lock. 
> 
> But I'm not sure whether there are such requirements for driver
> attributes:
> 
> some lock needs be grabbed in the driver attributes store callbacks, and
> the same lock also needs to be grabbed during driver unregister. 

In this case, the lock does _not_ need to be grabbed during driver 
unregister.  The driver grabs the lock, but it doesn't need to.

> If we have such requirements currently or in the future, I think they
> could all be solved by breaking active protection after get the module
> reference.

No!  That would be very bad.

Unloading modules is quite different from unbinding drivers.  After the
driver is unbound, its attribute callback routines can continue to run.  
But after a driver module has been unloaded, its attribute callback 
routines _cannot_ run because they aren't present in memory any more.

If we allowed a module to be unloaded while one of its callbacks was 
running (because active protection was broken), imagine what would 
happen...

Alan Stern

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