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

[Dmitry Torokhov]

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@xxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxx>
> >> >
> >> > Signed-off-by: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>
> >> > ---
> >> >  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

So no error handling whatsoever, as expected...

> 
> > If not will you be at Plumbers?
> 
> Yes.

Me too.

> 
> > Maybe we shoudl talk about this as although
> > ChromeOS already likely already jumped on async probe we should address a
> > way forward and path forward for other distributions and I don't think anyone
> > is looking too much into it. async probe came to Linux for two reasons:
> >
> >  * chromeos wanting it
> >  * an incorrect systemd assumption on how the driver core works
> >
> > So long term we still need to address the systemd approach, are they going
> > to be defaulting now to async probe for all modules? How about for built-ins?
> >
> > We should talk about this and maybe at plumbers.
> >
> >> Splitting the "initial probe" from
> >> the "manual probe" case seems like a recipe for confusion.
> >
> > If you can come up with pros / cons on both strategies it'd be
> > valuable.
> 
> The problem I ran into was needing to remove devices that still had
> yet to be probed and not being able to use registration completion vs
> the device_lock() to effectively synchronize the sub-system.

Why do you need to "synchronize the sub-system"? The asynchronous
probing should be transparent to the driver. Just unregister the device
(or the driver) and driver core will ensure that probe() is not in
flight.

Confused.

-- 
Dmitry
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