linux-kernel - Re: [PATCH] Driver core: fix race in dev_driver

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Message-ID: <Pine.LNX.4.44L0.0912041702510.2616-100000@iolanthe.rowland.org>
Date:	Fri, 4 Dec 2009 17:07:11 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Oliver Neukum <oliver@...kum.org>
cc:	Greg KH <gregkh@...e.de>, <stable@...nel.org>,
	Rickard Bellini <rickard.bellini@...csson.com>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	Torgny Johansson <torgny.johansson@...csson.com>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Driver core: fix race in dev_driver_string

On Fri, 4 Dec 2009, Oliver Neukum wrote:

> OK, yes. It's a bad example. However this is tricky.
> 
> This is a bug then:
> 
> mutex_lock(...);
> 
> if (instance->error) {
> 	rv = instance->error;
> 	instance->error  = 0;
> 	dev_dbg(instance->dev,...);

Unless you can guarantee at this point that instance->dev isn't stale, 
it is indeed a bug.

> 	goto err_out;
> }
> 
> rv = -ENODEV;
> if (instance->disconnected)
> 	goto err_out;
> 
> > One approach is to set instance->dev to NULL in disconnect().  That
> > wouldn't do much good for your dev_dbg(), though.  A better solution is
> > to refcount the instance->dev pointer: Take a reference to the device
> > when setting instance->dev and drop it when clearing instance->dev (or
> > when instance is freed).
> 
> That would mean that I am forced to adopt refcounting just to print
> something. This seems very inelegant.

What can I say?  When the something you want to print can be
deallocated at any time, there isn't much choice.

Maybe reference counting is inelegant; it depends on your point of
view.  Can you think of a more elegant way to make sure that a pointer
isn't stale?

Alan Stern

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