linux-kernel - Re: [PATCH 04/14] memstick: core: fix device

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Date:	Tue, 21 Sep 2010 15:49:47 -0700
From:	Greg KH <greg@...ah.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Kay Sievers <kay.sievers@...y.org>
Cc:	Vasiliy Kulikov <segooon@...il.com>,
	kernel-janitors@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	Jiri Slaby <jirislaby@...il.com>, linux-kernel@...r.kernel.org,
	James Bottomley <James.Bottomley@...e.de>,
	Dan Carpenter <error27@...il.com>,
	Boaz Harrosh <bharrosh@...asas.com>
Subject: Re: [PATCH 04/14] memstick: core: fix device_register() error
 handling

On Tue, Sep 21, 2010 at 03:20:31PM -0700, Andrew Morton wrote:
> On Sun, 19 Sep 2010 16:54:49 +0400
> Vasiliy Kulikov <segooon@...il.com> wrote:
> 
> > If device_register() fails then call put_device().
> > See comment to device_register.
> > 
> > Signed-off-by: Vasiliy Kulikov <segooon@...il.com>
> > ---
> >  compile tested.
> > 
> >  drivers/memstick/core/memstick.c |    1 +
> >  1 files changed, 1 insertions(+), 0 deletions(-)
> > 
> > diff --git a/drivers/memstick/core/memstick.c b/drivers/memstick/core/memstick.c
> > index c00fe82..4303b7e 100644
> > --- a/drivers/memstick/core/memstick.c
> > +++ b/drivers/memstick/core/memstick.c
> > @@ -465,6 +465,7 @@ static void memstick_check(struct work_struct *work)
> >  		if (!host->card) {
> >  			host->card = card;
> >  			if (device_register(&card->dev)) {
> > +				put_device(&card->dev);
> >  				kfree(host->card);
> >  				host->card = NULL;
> >  			}
> 
> A failed device_register() takes a bogus ref on the not-registered
> device?  It's no surprise that people are getting this wrong.  
> 
> The principle of least surprise says: fix device_register()!

One might think that, but it's a bit more difficult.

How does device_register know it should destroy the device if it fails?

Here's how it works:
 - device_register is just a wrapper around device_initialize() and
   device_add()
     - device_initialize() can't do anything wrong, so it's safe, BUT,
       at this point in time, the reference for the device is
       incremented, so any caller must now drop the reference and
       properly free stuff.
     - device_add() does a lot.

Hm, I guess, because we "know" in device_register() that we must drop
something if device_add() fails, then I guess it's not being consistant
with it's own calls...

So, something as simple as this?


diff --git a/drivers/base/core.c b/drivers/base/core.c
index d1b2c9a..4ba8599 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -1084,14 +1084,16 @@ name_error:
  * have a clearly defined need to use and refcount the device
  * before it is added to the hierarchy.
  *
- * NOTE: _Never_ directly free @dev after calling this function, even
- * if it returned an error! Always use put_device() to give up the
- * reference initialized in this function instead.
  */
 int device_register(struct device *dev)
 {
+	int retval;
+
 	device_initialize(dev);
-	return device_add(dev);
+	retval = device_add(dev);
+	if (retval)
+		put_device(dev);
+	return retval;
 }
 
 /**
     	


Kay, what am I missing here, why can't we just do this?  Hm, the
side-affect might be that if device_register() fails, NO ONE had better
touch that device again, as it might have just been freed from the
system.  I wonder if that will cause problems...

thanks,

greg k-h
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