Date:	Thu, 15 May 2008 13:40:40 -0700
From:	Greg KH <greg@...ah.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	torvalds@...ux-foundation.org, ajones@...erbed.com,
	a.p.zijlstra@...llo.nl, mszeredi@...e.cz,
	akpm@...ux-foundation.org, kay.sievers@...y.org,
	trond.myklebust@....uio.no, linux-kernel@...r.kernel.org
Subject: Re: [BUG] mm: bdi: export BDI attributes in sysfs

On Thu, May 15, 2008 at 01:37:48PM -0700, Greg KH wrote:
> On Thu, May 15, 2008 at 09:27:50PM +0200, Miklos Szeredi wrote:
> > > On Thu, 15 May 2008, Miklos Szeredi wrote:
> > > > 
> > > > This is not meant as a final solution, I'm sure Greg or Kay can help
> > > > find a better solution.
> > > 
> > > Yeah, don't do this:
> > > 
> > > > +static struct backing_dev_info *dev_get_bdi(struct device *dev)
> > > > +{
> > > > +	mutex_lock(&bdi_dev_mutex);
> > > > +	mutex_unlock(&bdi_dev_mutex);
> > > > +
> > > > +	return dev_get_drvdata(dev);
> > > > +}
> > > 
> > > This kind of serialization can often hide bugs, and in some cases even 
> > > make them go away (if the caller of the function means that the device is 
> > > pinned and the tear-down cannot happen, for example), but it's really 
> > > really bad form.
> > 
> > Yeah, I know.
> > 
> > > In order to use locking in a repeatable manner that is easy to think 
> > > about, you really need to *keep* the lock until you've stopped using the 
> > > data (or have dereferenced it into a stable form - eg maybe accessing the 
> > > pointer itself needs locking, but some individual data read _off_ the 
> > > pointer does not).
> > > 
> > > So the above kind of "get and release the lock" does obviously serialize 
> > > wrt others that hold the lock, but it's still wrong.
> > > 
> > > >  static ssize_t read_ahead_kb_store(struct device *dev,
> > > >  				  struct device_attribute *attr,
> > > >  				  const char *buf, size_t count)
> > > >  {
> > > > -	struct backing_dev_info *bdi = dev_get_drvdata(dev);
> > > > +	struct backing_dev_info *bdi = dev_get_bdi(dev);
> > > >  	char *end;
> > > >  	unsigned long read_ahead_kb;
> > > >  	ssize_t ret = -EINVAL;
> > > 
> > > You should just get the lock in the routines that acually use this thing.
> > > 
> > > Or, if the "struct backing_dev_info *" pointer itself is stable, and it 
> > > really is just the access from "struct device" that needs protection, then 
> > > at the very least it should have been
> > 
> > Actually nothing should need protection.  The only problem AFAICS is
> > that the device_create()/dev_set_drvdata() interface is racy: somebody
> > can come in after the device has been created but before drvdata has
> > been set, and then we are in trouble.
> 
> Then that needs to be fixed in the code that registered the device
> itself.  The driver core knows nothing about this at all.  Is this
> something in the block layer?
> 
> > I'm quite sure this is not the only place in the kernel where this
> > would be an issue, that's why I expect the sysfs guys to have some
> > sort of alternative solution, that doesn't necessarily involve adding
> > a new mutex.
> 
> It should be fixed in the bus/subsystem that is creating the device, the
> pointer must be set up before device_register() is called (or
> device_add()).

Oh nevermind, Linus is right.  We need to just add another parameter to
device_create() for this field.  For now I can make up a
device_create_drvdata() like Linus suggested.  Give me a few minutes...

thanks,

greg k-h
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