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

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Date:	Tue, 7 Jul 2015 10:45:16 +0200
From:	Tom Gundersen <teg@...m.no>
To:	"Luis R. Rodriguez" <mcgrof@...e.com>
Cc:	Dan Williams <dan.j.williams@...el.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	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 Tue, Jul 7, 2015 at 1:23 AM, Luis R. Rodriguez <mcgrof@...e.com> 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.
> This however does not address systemd's assumption over detachment of module
> load and probe. The inherent problem there was the timeout implemented and
> carried in systemd over the worker that uses modlib to load modules. Upon
> review the code was complex enough already and surely increasing the timeout
> helps but that doesn't address all issues with a general timeout in place.
> At SUSE we ended up not using a timeout for kmod built-in commands. That
> leaves the original timeout purpose in place. The code for async probe was
> not put in the kernel though but since its now upstream we should be able
> to replace that userspace systemd work around with async probe, but systemd
> folks would need to decide what they want to do. For full gory details of
> this refer to:
>
> https://bugzilla.novell.com/show_bug.cgi?id=889297

FTR, this does not appear to be public, so I was not able to see it.

>> > If not will you be at Plumbers?
>>
>> Yes.
>
> Great, Tom, Dmitry, will you be at Plumbers?

Sadly, I won't make plumbers this year.

Cheers,

Tom
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