linux-kernel - Re: [PATCH v3] drivers/base/core: refcount kobject and bus on device attribute read / store

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Date:   Wed, 23 Jun 2021 18:28:05 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     rafael@...nel.org, jeyu@...nel.org, ngupta@...are.org,
        sergey.senozhatsky.work@...il.com, minchan@...nel.org,
        axboe@...nel.dk, mbenes@...e.com, jpoimboe@...hat.com,
        tglx@...utronix.de, keescook@...omium.org, jikos@...nel.org,
        rostedt@...dmis.org, peterz@...radead.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] drivers/base/core: refcount kobject and bus on device
 attribute read / store

On Wed, Jun 23, 2021 at 09:14:34AM -0700, Luis Chamberlain wrote:
> On Wed, Jun 23, 2021 at 10:32:53AM +0200, Greg KH wrote:
> > On Tue, Jun 22, 2021 at 05:36:30PM -0700, Luis Chamberlain wrote:
> > > diff --git a/drivers/base/core.c b/drivers/base/core.c
> > > index 4a8bf8cda52b..f69aa040b56d 100644
> > > --- a/drivers/base/core.c
> > > +++ b/drivers/base/core.c
> > > @@ -2042,28 +2042,56 @@ EXPORT_SYMBOL(dev_driver_string);
> > >  static ssize_t dev_attr_show(struct kobject *kobj, struct attribute *attr,
> > >  			     char *buf)
> > >  {
> > > -	struct device_attribute *dev_attr = to_dev_attr(attr);
> > > -	struct device *dev = kobj_to_dev(kobj);
> > > +	struct device_attribute *dev_attr;
> > > +	struct device *dev;
> > > +	struct bus_type *bus = NULL;
> > >  	ssize_t ret = -EIO;
> > >  
> > > +	dev = get_device(kobj_to_dev(kobj));
> > > +	if (dev->bus) {
> > 
> > No need to test for this, right?
> 
> dev_uevent() checks for dev->bus, so I thought that was a clear
> indication this isn't always set.
> 
> > 
> > > +		bus = bus_get(dev->bus);
> > > +		if (!bus)
> > > +			goto out;

The point is that even if dev->bus is NULL, then bus_get(NULL) is NULL.
That's the only way that bus_get() can return NULL, which means this
check too is not needed.

> > >  	if (dev_attr->show)
> > >  		ret = dev_attr->show(dev, dev_attr, buf);
> > >  	if (ret >= (ssize_t)PAGE_SIZE) {
> > >  		printk("dev_attr_show: %pS returned bad count\n",
> > >  				dev_attr->show);
> > >  	}
> > > +
> > > +	bus_put(bus);
> > 
> > You are incrementing the bus, which is nice, but I do not understand why
> > it is needed.  What is causing the bus to go away _before_ the devices
> > are going away?  Busses almost never are removed from the system, and if
> > they are, all devices associated with them are removed first.  So I do
> > not think you need to increment anything with that here.
> 
> You tell me. It was your suggestion as a replacement for the type
> specific lock, in the zram case, its a block device so I was using
> bdgrab().

I did?  Sorry, I do not remember, but this is not a lock, nor does it
protect anything.

I'll respond to the rest later...

greg k-h