Re: [driver-core PATCH v6 4/9] driver core: Move async_synchronize_full call

[Alexander Duyck]

On Mon, 2018-11-26 at 18:11 -0800, Dan Williams wrote:
> On Thu, Nov 8, 2018 at 10:07 AM Alexander Duyck
> <alexander.h.duyck@xxxxxxxxxxxxxxx> wrote:
> > 
> > Move the async_synchronize_full call out of __device_release_driver and
> > into driver_detach.
> > 
> > The idea behind this is that the async_synchronize_full call will only
> > guarantee that any existing async operations are flushed. This doesn't do
> > anything to guarantee that a hotplug event that may occur while we are
> > doing the release of the driver will not be asynchronously scheduled.
> > 
> > By moving this into the driver_detach path we can avoid potential deadlocks
> > as we aren't holding the device lock at this point and we should not have
> > the driver we want to flush loaded so the flush will take care of any
> > asynchronous events the driver we are detaching might have scheduled.
> > 
> 
> What problem is this patch solving in practice, because if there were
> drivers issuing async work from probe they would need to be
> responsible for flushing it themselves. That said it seems broken that
> the async probing infrastructure takes the device_lock inside
> async_schedule and then holds the lock when calling
> async_syncrhonize_full. Is it just luck that this hasn't caused
> deadlocks in practice?

My understanding is that it has caused some deadlocks. There was
another patch set that Bart Van Assche had submitted that was
addressing this. I just tweaked my patch set to address both the issues
he had seen as well as the performance improvements included in my
original patch set.

> Given that the device_lock is hidden from lockdep I think it would be
> helpful to have a custom lock_map_acquire() setup, similar to the
> workqueue core, to try to keep the locking rules enforced /
> documented.
> 
> The only documentation I can find for async-probe deadlock avoidance
> is the comment block in do_init_module() for async_probe_requested.

Would it make sense to just add any lockdep or deadlock documentation
as a seperate patch? I can work on it but I am not sure it makes sense
to add to this patch since there is a chance this one will need to be
backported to stable at some point.

> Stepping back a bit, does this patch have anything to do with the
> performance improvement, or is it a separate "by the way I also found
> this" kind of patch?

This is more of a seperate "by the way" type of patch based on the
discussion Bart and I had about how to best address the issue. There
may be some improvement since we only call async_synchronize_full once
and only when we are removing the driver, but I don't think it would be
very noticable.