linux-kernel - Re: [RFC v1 0/3] driver-core: add asynch module loading support

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Date:	Sun, 31 Aug 2014 07:38:22 -0400
From:	Tejun Heo <tj@...nel.org>
To:	David Herrmann <dh.herrmann@...il.com>
Cc:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>, falcon@...zu.com,
	Takashi Iwai <tiwai@...e.de>, arjan@...ux.intel.com,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	penguin-kernel@...ove.sakura.ne.jp,
	Joseph Salisbury <joseph.salisbury@...onical.com>,
	bpoirier@...e.de, "Luis R. Rodriguez" <mcgrof@...e.com>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [RFC v1 0/3] driver-core: add asynch module loading support

(cc'ing Rusty for module loading)

Hello,

On Sun, Aug 31, 2014 at 01:25:03PM +0200, David Herrmann wrote:
> On Sun, Aug 31, 2014 at 1:02 PM, Tejun Heo <tj@...nel.org> wrote:
> > @@ -689,9 +704,23 @@ int bus_add_driver(struct device_driver *drv)
> >
> >         klist_add_tail(&priv->knode_bus, &bus->p->klist_drivers);
> >         if (drv->bus->p->drivers_autoprobe) {
> > -               error = driver_attach(drv);
> > -               if (error)
> > -                       goto out_unregister;
> > +               struct driver_attach_work *daw;

Oops, this probably should go inside the below if block.

> > +
> > +               if (drv->owner) {
> > +                       daw = kzalloc(sizeof(*daw), GFP_KERNEL);
> > +                       if (!daw) {
> > +                               error = -ENOMEM;
> > +                               goto out_unregister;
> > +                       }
> > +
> > +                       INIT_WORK(&daw->work, driver_attach_workfn);
> > +                       daw->driver = drv;
> > +                       queue_work(driver_attach_wq, &daw->work);
> 
> Doesn't this break on-demand cdev initialization? We currently call
> request_module() on open() for unclaimed major/minor combinations. If
> driver_attach() is no longer part of module_init(), there is no
> guarantee the driver created the cdev before request_module() returns.

Right, yeah, this really looks like something we'd need to switch per
insmod instance.  It looks like driver core needs a generic parameter
to tell it to whether drive probing asynchronously to module loading
or not.  Maybe we can add a generic driver param like
"driver_async_probe"?

> I actually like this "deferred attach" approach, so this is not meant
> as counter-argument. We just need to make sure to have a notion of
> "settled modules" so we know how long to wait after loading a module.

Another way could be making module-generic pollable file in the
module's sysfs dir to indicate "full init completion", which would map
to probing completion for drivers; however, given that we need to keep
the synchronous behavior by default for compatibility, I don't think
that buys us much.

Thanks.

-- 
tejun
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