linux-kernel - Re: [PATCH 2/8] driver-core: add asynchronous probing support for drivers

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Date:	Wed, 8 Jul 2015 17:49:03 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	"Luis R. Rodriguez" <mcgrof@...e.com>, Tom Gundersen <teg@...m.no>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Tejun Heo <tj@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Olof Johansson <olof@...om.net>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Subject: Re: [PATCH 2/8] driver-core: add asynchronous probing support for
 drivers

On Wed, Jul 08, 2015 at 05:36:04PM -0700, Dan Williams wrote:
> On Mon, Jul 6, 2015 at 4:40 PM, Dmitry Torokhov
> <dmitry.torokhov@...il.com> wrote:
> > On Tue, Jul 07, 2015 at 01:23:15AM +0200, Luis R. Rodriguez wrote:
> >> On Sat, Jul 04, 2015 at 07:09:19AM -0700, Dan Williams wrote:
> >> > On Fri, Jul 3, 2015 at 11:30 AM, Luis R. Rodriguez <mcgrof@...e.com> wrote:
> >> > > On Sat, Jun 27, 2015 at 04:45:25PM -0700, Dan Williams wrote:
> >> > >> On Mon, Mar 30, 2015 at 4:20 PM, Dmitry Torokhov
> >> > >> <dmitry.torokhov@...il.com> wrote:
> >> > >> > Some devices take a long time when initializing, and not all drivers are
> >> > >> > suited to initialize their devices when they are open. For example,
> >> > >> > input drivers need to interrogate their devices in order to publish
> >> > >> > device's capabilities before userspace will open them. When such drivers
> >> > >> > are compiled into kernel they may stall entire kernel initialization.
> >> > >> >
> >> > >> > This change allows drivers request for their probe functions to be
> >> > >> > called asynchronously during driver and device registration (manual
> >> > >> > binding is still synchronous). Because async_schedule is used to perform
> >> > >> > asynchronous calls module loading will still wait for the probing to
> >> > >> > complete.
> >> > >> >
> >> > >> > Note that the end goal is to make the probing asynchronous by default,
> >> > >> > so annotating drivers with PROBE_PREFER_ASYNCHRONOUS is a temporary
> >> > >> > measure that allows us to speed up boot process while we validating and
> >> > >> > fixing the rest of the drivers and preparing userspace.
> >> > >> >
> >> > >> > This change is based on earlier patch by "Luis R. Rodriguez"
> >> > >> > <mcgrof@...e.com>
> >> > >> >
> >> > >> > Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
> >> > >> > ---
> >> > >> >  drivers/base/base.h    |   1 +
> >> > >> >  drivers/base/bus.c     |  31 +++++++---
> >> > >> >  drivers/base/dd.c      | 149 ++++++++++++++++++++++++++++++++++++++++++-------
> >> > >> >  include/linux/device.h |  28 ++++++++++
> >> > >> >  4 files changed, 182 insertions(+), 27 deletions(-)
> >> > >>
> >> > >> Just noticed this patch.  It caught my eye because I had a hard time
> >> > >> getting an open coded implementation of asynchronous probing to work
> >> > >> in the new libnvdimm subsystem.  Especially the messy races of tearing
> >> > >> things down while probing is still in flight.  I ended up implementing
> >> > >> asynchronous device registration which eliminated a lot of complexity
> >> > >> and of course the bugs.  In general I tend to think that async
> >> > >> registration is less risky than async probe since it keeps wider
> >> > >> portions of the traditional device model synchronous
> >> > >
> >> > > but its not see -DEFER_PROBE even before async probe.
> >> >
> >> > Except in that case you know probe has been seen by the driver at
> >> > least once.  So I see that as less of a surprise, but point taken.
> >> >
> >> > >> and leverages the
> >> > >> fact that the device model is already well prepared for asynchronous
> >> > >> arrival of devices due to hotplug.
> >> > >
> >> > > I think this sounds reasonable, do you have your code upstream or posted?
> >> >
> >> > Yes, see nd_device_register() in drivers/nvdimm/bus.c
> >>
> >> It should be I think rather easy for Dmitry to see if he can convert this input
> >> driver (not yet upstream) to this API and see if the same issues are fixed.
> >
> > No, I would rather not as it means we lose error handling on device
> > registration.
> >
> 
> I think this is a red herring as I don't see how async probing is any
> better at handling device registration errors.  The error is logged
> and "handled" by the fact that a device fails to appear, what other
> action would you take?  In fact libnvdimm does detect registration
> failures and reports that in a parent device attribute (at least for a
> region device and their namespace child devices).

What is libnvdimm behavior if you try to unload a module that tries to
register a device but it failed? Memory leak or crash, right?

-- 
Dmitry
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