Re: lockdep warning in urb.c:363 usb_submit_urb

Alan Stern
2020-04-06T20:25:10Z

On Mon, 6 Apr 2020, Rafael J. Wysocki wrote:

> In the meantime I have created a git branch with changes to simplify the code,
> rename some things and clarify the documentation a bit:
>
> git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git \
> pm-sleep-core
>
> (https://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git/log/?h=pm-sleep-core
> for web access).
>
> I'm going to post these changes as patches soon.

All right, those are some significant changes. It'll take me a little
while to absorb them.

> On Friday, April 3, 2020 10:15:09 PM CEST Alan Stern wrote:

> > Let's put it like this: The resume-side callbacks should have the
> > overall effect of bringing the device back to its initial state, with
> > the following exceptions and complications:
> >
> > Unless SMART_SUSPEND and LEAVE_SUSPEND are both set, a device
> > that was in runtime suspend before the suspend_late phase
> > must end up being runtime-active after the matching RESUME.
> >
> > Unless SMART_SUSPEND is set, a device that was in runtime
> > suspend before the freeze_late phase must end up being
> > runtime-active after the matching THAW.
>
> Correct.
>
> > [I'm not so sure about this. Wouldn't it make more sense to treat
> > _every_ device as though SMART_SUSPEND was set for FREEZE/THAW
> > transitions, and require subsystems to do the same?]
>
> Drivers may expect devices to be runtime-active when their suspend
> callbacks are invoked unless they set SMART_SUSPEND. IOW, without
> SMART_SUSPEND set the device should not be left in runtime suspend
> during system-wide suspend at all unless direct-complete is applied
> to it.

[Let's confine this discussion to the not-direct-complete case.]

Okay, say that SMART_SUSPEND isn't set and the device is initially
runtime-suspended. Since the core knows all this, shouldn't the core
then call pm_runtime_resume() immediately before ->suspend? Why leave
this up to subsystems or drivers (which can easily get it wrong --
not to mention all the code duplication it would require)?

Also, doesn't it make sense for some subsystems or drivers to want
their devices to remain in runtime suspend throughout a FREEZE/THAW
transition but not throughout a SUSPEND/RESUME transition? With only a
single SMART_SUSPEND flag, how can we accomodate this desire?

Finally, my description above says that LEAVE_SUSPENDED matters for
SUSPEND/RESUME but not for FREEZE/THAW. Is that really what you have
in mind?

> > After RESTORE, _every_ device must end up being runtime
> > active.
>
> Correct.
>
> > In general, each resume-side callback should undo the effect
> > of the matching suspend-side callback. However, because of
> > the requirements mentioned in the preceding sentences,
> > sometimes a resume-side callback will be issued even though
> > the matching suspend-side callback was skipped -- i.e., when
> > a device that starts out runtime-suspended ends up being
> > runtime-active.
> >
> > How does that sound?
>
> It is correct, but in general the other way around is possible too.
> That is, a suspend-side callback may be issued without the matching
> resume-side one and the device's PM runtime status may be changed
> if LEAVE_SUSPENDED is set and SMART_SUSPEND is unset.

This is inconsistent with what I wrote above (the "Unless SMART_SUSPEND
and LEAVE_SUSPENDED are both set" part). Are you saying that text
should be changed?

> > Are you certain you want the subsystem callback to be responsible for
> > setting the runtime status to "active"? Isn't this an example of
> > something the core could do in order to help simplify subsystems?
>
> The rationale here is that whoever decides whether or not to skip the
> driver-level callbacks, should also set the PM-runtime status of the
> device to match that decision.

Well, that's not really a fair description. The decision about
skipping driver-level callbacks is being made right here, by us, now.
(Or if you prefer, by the developers who originally added the
SMART_SUSPEND flag.) We require subsystems to obey the decisions being
outlined in this discussion.

Given that fact, this is again a case of having the core do something
rather than forcing subsystems/drivers to do it (possibly getting it
wrong and certainly creating a lot of code duplication).

If a subsystem really wants to override our decision, it can always
call pm_runtime_set_{active|suspended} to override the core's setting.

> > And this brings up another thing the core might do to help simplify
> > drivers and subsystems: If SMART_SUSPEND isn't set and the device is in
> > runtime suspend, couldn't the core do a pm_runtime_resume before
> > issuing the ->suspend or ->suspend_late callback?
>
> It could, but sometimes that is not desirable. Like when the drivver points its
> suspend callback to pm_runtime_force_suspend().

This seems to contradict what you wrote above: "Drivers may expect
devices to be runtime-active when their suspend callbacks are invoked
unless they set SMART_SUSPEND. IOW, without SMART_SUSPEND set the
device should not be left in runtime suspend during system-wide suspend
at all unless direct-complete is applied to it."

If you stand by that statement then drivers should never point their
suspend callback to pm_runtime_force_suspend() unless they also set
SMART_SUSPEND.

Alan Stern