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

Dan Williams
2015-07-05T09:04:05Z

On Fri, Jul 3, 2015 at 11:30 AM, Luis R. Rodriguez <mcgrof@suse.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@gmail.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@suse.com>
>> >
>> > Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.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

> If not will you be at Plumbers?

Yes.

> 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.