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SubjectRe: [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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