Messages in this thread | | | Subject | Re: [driver-core PATCH v6 4/9] driver core: Move async_synchronize_full call | From | Alexander Duyck <> | Date | Tue, 27 Nov 2018 09:38:01 -0800 |
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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@linux.intel.com> 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.
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