On Wed, Oct 28, 2020 at 9:25 AM Michael Auchter <michael.auchter@xxxxxx> wrote: > > Hey Saravana, > > Thanks for taking the time to look into this! > > On Mon, Oct 26, 2020 at 12:10:33PM -0700, Saravana Kannan wrote: > > On Wed, Oct 21, 2020 at 2:02 PM Frank Rowand <frowand.list@xxxxxxxxx> wrote: > > > > > > Hi Saravana, > > > > > > Michael found an issue related to the removal of a devicetree node > > > which involves devlinks: > > > > > > On 10/14/20 2:36 PM, Michael Auchter wrote: > > > > After updating to v5.9, I've started seeing errors in the kernel log > > > > when using device tree overlays. Specifically, the problem seems to > > > > happen when removing a device tree overlay that contains two devices > > > > with some dependency between them (e.g., a device that provides a clock > > > > and a device that consumes that clock). Removing such an overlay results > > > > in: > > > > > > > > OF: ERROR: memory leak, expected refcount 1 instead of 2, of_node_get()/of_node_put() unbalanced - destroy > > > > OF: ERROR: memory leak, expected refcount 1 instead of 2, of_node_get()/of_node_put() unbalanced - destroy > > > > > > > > followed by hitting some REFCOUNT_WARNs in refcount.c > > > > > > > > In the first patch, I've included a unittest that can be used to > > > > reproduce this when built with CONFIG_OF_UNITTEST [1]. > > > > > > > > I believe the issue is caused by the cleanup performed when releasing > > > > the devlink device that's created to represent the dependency between > > > > devices. The devlink device has references to the consumer and supplier > > > > devices, which it drops in device_link_free; the devlink device's > > > > release callback calls device_link_free via call_srcu. > > > > > > > > When the overlay is being removed, all devices are removed, and > > > > eventually the release callback for the devlink device run, and > > > > schedules cleanup using call_srcu. Before device_link_free can and call > > > > put_device on the consumer/supplier, the rest of the overlay removal > > > > process runs, resulting in the error traces above. > > > > > > When a devicetree node in an overlay is removed, the remove code expects > > > all previous users of the related device to have done the appropriate put > > > of the device and to have no later references. > > > > > > As Michael described above, the devlink release callback defers the > > > put_device(). The cleanup via srcu was implemented in commit > > > 843e600b8a2b01463c4d873a90b2c2ea8033f1f6 "driver core: Fix sleeping > > > in invalid context during device link deletion" to solve yet another > > > issue. > > > > > > > > > > > > > > Patches 2 and 3 are an attempt at fixing this: call srcu_barrier to wait > > > > for any pending device_link_free's to execute before continuing on with > > > > the removal process. > > > > > > > > These patches resolve the issue, but probably not in the best way. In > > > > particular, it seems strange to need to leak details of devlinks into > > > > the device tree overlay code. So, I'd be curious to get some feedback or > > > > hear any other ideas for how to resolve this issue. > > > > > > I agree with Michael that adding an indirect call of srcu_barrier(&device_links_srcu) > > > into the devicetree overlay code is not an appropriate solution. > > > > I kind of see your point too. I wonder if the srcu_barrier() should > > happen inside like so: > > device_del() -> device_links_purge()->srcu_barrier() > > > > I don't know what contention the use of SRCUs in device links was > > trying to avoid, but I think the srcu_barrier() call path I suggested > > above shouldn't be a problem. If that fixes the issue, the best way to > > know if it's an issue is to send out a patch and see if Rafael has any > > problem with it :) > > I was able to test this by adding the srcu_barrier() at the end of > device_links_purge(), and that does seem to have fixed the issue. Thanks for testing my suggestion. If you send out a patch for that, I'd appreciated a Suggested-by: tag. > > > Is there some other way to fix the problem that 843e600b8a2b solves without > > > deferring the put_device() done by the devlink release callback? > > > > Ok I finally got some time to look into this closely. > > > > Even if you revert 843e600b8a2b, you'll see that device_link_free() > > (which drops the reference to the consumer and supplier devices) was > > scheduled to run when the SRCU clean up occurs. So I think this issue > > was present even before 843e600b8a2b, but commit 843e600b8a2b just > > made it more likely to hit this scenario because it introduces some > > delay in dropping the ref count of the supplier and consumer by going > > through the device link device's release path. So, I think this issue > > isn't related to 843e600b8a2b. > > > > As to why 843e600b8a2b had to be written to call call_srcu() from the > > device link device's release path, it's a mess of dependencies/delays: > > 1. The device link device is part of the struct device_link. So we > > can't free device_link before the device_link.link_dev refcount goes > > to 0. > > 2. But I can't assume device_link.link_dev's refcount will go to 0 as > > soon as I call put_device() on it because of > > CONFIG_DEBUG_KOBJECT_RELEASE which frees up the kobject after a random > > delay. > > 3. The use of SRCU also means I can't free device_link until the SRCU > > is cleaned up. > > > > Because of (1), (2) and (3), when the device_link_del() (or any of the > > other device link deletion APIs are called) I first have to do a > > put_device(device_link.link_dev) to make sure the device memory is no > > longer referenced, then trigger an SRCU clean up and then in the > > scheduled SRCU cleanup I can free struct device_link. And obviously, > > until struct device_link is ready to be freed up, I can't drop the > > reference to the supplier and consumer devices (as that old copy of > > device_link could be used by some code to refer to the supplier and > > consumer devices). > > > > Hope that helps explain the SRCU and device link device release dependencies. > > > > Also, even if this patch series is applied as is, I wonder if the > > current overlay code has a bug related to CONFIG_DEBUG_KOBJECT_RELEASE > > delaying the actual freeing of the device. Something to look into? > > I also tried enabling CONFIG_DEBUG_KOBJECT_RELEASE... with or without > the addition of srcu_barrier() to device_links_purge(), I can't boot > successfully when CONFIG_OF_UNITTEST=y && > CONFIG_DEBUG_KOBJECT_RELEASE=y: there are a ton of errors that result > from this combo. > > Disabling the unittests and booting with CONFIG_DEBUG_KOBJECT_RELEASE=y, > I _do_ still see the errors mentioned in my original message when > removing an overlay. So yeah, it does seem like there are some latent > issues here... Thanks for confirming my suspicion. I assume you see these errors even with the srcu_barrier() call? I'll leave this to Frank then :) -Saravana
