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

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Date:	Mon, 28 Apr 2014 09:58:52 +0800
From:	Li Zhong <zhong@...ux.vnet.ibm.com>
To:	Alan Stern <stern@...land.harvard.edu>
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, 2014-04-25 at 09:59 -0400, Alan Stern wrote:
> 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.

OK.

> 
> > 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...

I don't think the module could be unloaded after we increased the module
reference counter. 

Thanks, Zhong

> 
> Alan Stern
> 


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