Re: [PATCH v1] driver core: Take parent lock in async device attach

[Alan Stern <stern@rowland.harvard.edu>]

On Tue, Oct 06, 2026 at 03:29:48PM +0000, PETER Mario wrote:
> Hi Greg,
> 
> On 10/6/26 16:38, Greg Kroah-Hartman wrote:
> 
> > On Tue, Oct 06, 2026 at 01:23:35PM +0000, Mario Peter wrote:
> >> On buses with need_parent_lock set (only USB), probe() must run with the
> >> parent device locked. A synchronous attach after device_add() gets this
> >> from the caller, e.g. usb_set_configuration() holds the udev lock while
> >> adding interfaces. __device_attach_async_helper() runs the probe from an
> >> async worker instead, where that lock isn't held, and only takes
> >> device_lock(dev).
> >>
> >> With async probing enabled for USB drivers (driver_async_probe=*,
> >> module.async_probe=1), hub_probe() of a multi-TT hub then races with
> >> usb_set_configuration() and both create the interface's endpoint
> >> devices. On an i.MX8MM board this hit 11 of 100 boots:
> >>
> >>   sysfs: cannot create duplicate filename '.../1-1/1-1:1.0/ep_81'
> >>   Call trace:
> >>    sysfs_warn_dup
> >>    usb_create_ep_devs
> >>    create_intf_ep_devs
> >>    usb_set_interface
> >>    hub_probe
> >>    usb_probe_interface
> >>    really_probe
> >>    __device_attach_async_helper
> >>    async_run_entry_fn
> >>
> >> Take the parent lock there as well, like __driver_attach_async_helper()
> >> does, and move __device_driver_lock/unlock() up for that. With this the
> >> warning was gone in 300 boots.
> >>
> >> Fixes: 765230b5f084 ("driver-core: add asynchronous probing support for drivers")
> >> Assisted-by: LLM
> >> Signed-off-by: Mario Peter <mario.peter@leica-geosystems.com>
> >> ---
> >> Seen and tested on 6.16.y, which has the same code here. On mainline
> >> only build-tested.
> > 
> > Please verify this on the latest tree, 6.16.y is _VERY_ old and
> > obsolete, lots has changed in the year it was released.
> 
> Moving this board to the latest kernel for a test isn't that easy, as
> its board support isn't upstream.

It should be fairly simple to run the verification on a standard PC 
using an up-to-date kernel.  Nothing in the bug description or fix is 
specific to i.MX8MM.

> But the affected code is still the
> same in v7.3-rc6. In dd.c, __device_attach_async_helper(),
> __device_attach() and __driver_attach_async_helper() are unchanged
> since v6.16. On the USB side, usb_set_interface() and
> create_intf_ep_devs() are unchanged, and usb_set_configuration() and
> hub_configure() only got the kmalloc_obj() conversions.
> 
> >> Not covered: deferred_probe_work_func() also re-probes via
> >> __device_attach() without the parent lock.
> >>
> >> Analysis and patch done with the help of Claude Code.
> > 
> > This feels really wrong, what bus is the host controller on for these?
> 
> The platform bus, it's the ChipIdea controller of the i.MX8MM:
>    /sys/devices/platform/soc@0/32c00000.bus/32e50000.usb/ci_hdrc.1/usb1/1-1/1-1:1.0
> 
> 1-1 is an onboard USB2514 hub (multi-TT). The host controller isn't

Does the fact that the onboard hub is multi-TT have any connection with 
the bug?  Not as far as I can see -- but I had to waste a minute 
thinking about it.  If you agree, please remove that irrelevant detail 
from the patch description.

> part of the race, both sides are in the USB core, for that one hub:
>   
> - usb_generic_driver_probe() of 1-1 calls usb_set_configuration(),
>   which holds the 1-1 lock, does device_add() for 1-1:1.0 and then
>   create_intf_ep_devs() for it.
>      
> - device_add() only queues the probe of 1-1:1.0. The async worker runs
>   hub_probe() -> usb_set_interface(hdev, 0, 1) -> create_intf_ep_devs()
>   with only the 1-1:1.0 lock held.
>      
> create_intf_ep_devs() checks and sets intf->ep_devs_created without a
> lock of its own, so both create ep_81. With a synchronous probe,
> hub_probe() runs inside device_add() with the 1-1 lock held, and this
> can't happen.

You should describe this race in more detail (like you just did here) in 
the patch description.  It will help explain exactly what it is you are 
fixing.

> The USB core relies on that lock. need_parent_lock is documented as
> "When probing or removing a device on this bus, the device core should
> lock the device's parent", and usb_driver_claim_interface() says
> "Callers must own the device lock, so driver probe() entries don't need
> extra locking". __driver_attach_async_helper() takes the parent lock,
> __device_attach_async_helper() doesn't.

FWIW, I agree that this is a real bug and your solution is the right 
approach for fixing it.

Alan Stern