Date:	Thu, 19 Apr 2007 14:39:40 -0400
From:	"Dmitry Torokhov" <dmitry.torokhov@...il.com>
To:	"Alan Stern" <stern@...land.harvard.edu>
Cc:	"Tejun Heo" <htejun@...il.com>,
	"Cornelia Huck" <cornelia.huck@...ibm.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"Greg K-H" <greg@...ah.com>,
	"Rusty Russell" <rusty@...tcorp.com.au>
Subject: Re: [PATCH RFD] alternative kobject release wait mechanism

On 4/19/07, Alan Stern <stern@...land.harvard.edu> wrote:
> On Wed, 18 Apr 2007, Dmitry Torokhov wrote:
>
> > I am still do not understand why this is needed. Would it not be
> > simplier just to use a reference to struct device instead of embedding
> > it in a larger structure if their lifetimes are different and one does
> > not have a subsystem that takes care of releasing logic.
> >
> >
> > Pretty much drivers have 2 options:
> >
> > struct my_device {
> >         void *private_data;
> >         struct device dev;
> > };
>
> Actually people use dev_[gs]et_drvdata() instead of a separate
> private_data pointer.  That way there's no need for the my_device
> container.
>

No, drvdata belongs to driver that is bound to a device. We are
talking about private data created and managed by driver that provides
device.

> > In this case ->release must live in a subsystem code; individual
> > drivers kfree(my_dev->private) and do any additional cleanup after
> > calling device_unregister(&my_dev->dev);
>
> That doesn't sound right.  Generally the call to device_unregister() and
> the release method live in the same module.  Maybe you meant to say
> individual drivers kfree(my_dev->private_data) and do any additional
> cleanup in their remove() routine.

Again, if we are talking about driver bound to a device then devices
->release() is irrelevant. If we are talking about driver that created
device then driver's ->remove() is irrelevant.

>
> This approach seems dangerous.  Suppose there's mutex embedded in
> my_dev->private_data, and suppose some other thread is blocked waiting on
> that mutex when remove() is called.  That other thread will then oops when
> my_dev->private_data is deallocated.

What other thread? I suppose it is module-local thread or
subsystem-local thread. Let's that particular subsystem take care of
it's own data and use its own ->release() when it is ready. What I
mean is there is no need to perform clean-up at once; every layer can
do its own cleanup.

>
> > Second option:
> >
> > struct my_device {
> >         type member1;
> >         type member2;
> >
> >        struct device *dev;
> > };
> >
> > dev is coming from _device_create(). Driver core takes care of
> > releasing dev structure; driver does cleanup of my_device.
>
> Lots of drivers create devices dynamically without using device_create().
>
> More to the point, how does the driver clean up my_device?  It probably
> has a reference count somewhere in my_device, especially if my_device is
> shared with other threads or other drivers.  We then face exactly the same
> problem: What happens if the driver's module is unloaded before all the
> references to my_device are gone?
>

This is up to subsystem to ensure that it does not access dead devices.

> > With current sysfs orphaning attributes upon removal request there is
> > no issue of accessing driver-private data through references obtained
> > via ether embedded or referenced dev structure so everything is fine.
>
> Not so.  There are other pathways besides sysfs which can cause a driver
> to access its data structures.
>

Which ones? These needs to be identified and treated with "immediate
disconnect" that you advocated earlier. Once active users of device's
services are gone you can zap it.

-- 
Dmitry
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