Re: [PATCH v2] thunderbolt: Do not RPM complete unrelated subtrees on unplug

[Mika Westerberg <mika.westerberg@linux.intel.com>]

Hi,

On Sun, Oct 04, 2026 at 11:13:50AM -0600, Jonathan Gopel wrote:
> When a Thunderbolt device is unplugged, that switch and every switch
> subsequent to it on the bus device list is `complete()`d, not just its
> child devices. In the case of multiple domains, it may cause, eg, an
> attempt to `complete()` the uninitialized domain 1 root switch RPM
> completion on unplug of a switch from domain 0 (though ordering is not
> guaranteed numerically sorted).
> 
> I initially noticed this issue on a Dell XPS9720 when the system would
> seem to sporadically not wake after unplugging a Thunderbolt dock. I
> tried multiple kernel versions including v7.2.7 and v7.3-rc4 to see if
> that resolved the issue, but the issue persisted. Eventually I found
> that I could consistently generate a kernel Oops reporting a page fault
> at 0xffff_ffff_ffff_fff8 on unplug from one side of the laptop and went
> bug hunting. I became suspicious of the `bus_for_each_dev()` walk that I
> ultimately ended up changing. To confirm this issue, I did an eBPF trace
> of `bus_for_each_dev()`, `complete_rpm()`, and `complete()` while
> unplugging in a variety of situations and found that an unplug from
> domain 0 tries to `complete_rpm()` domain 1's switch, whose RPM
> completion is not initialized and thus is holding a null wait-list
> pointer, triggering the page fault.
> 
> Here are the key sections of that eBPF trace. Note that HEAD denotes the
> completion's own wait-list HEAD:
> 
> ```
> RESCAN_ENTER domain=0
> 
> COMPLETE rescan_domain=0 target=0-1 target_domain=0
>          route=0x1 parent=0-0 unplugged=1
> COMPLETION done=0 head=HEAD next=HEAD prev=HEAD
> 
> BUS_WALK domain=0 start=0-1 callback=complete_rpm
> CALLBACK_ENTER rescan_domain=0 dev=1-0 parent=domain1 is_switch=1
> 
> COMPLETE rescan_domain=0 target=1-0 target_domain=1
>          route=0x0 parent=domain1 unplugged=0
> COMPLETION done=0 head=HEAD next=0 prev=0
> 
>         complete+5
>         complete_rpm+43
>         bus_for_each_dev+133
>         icm_free_unplugged_children+250
>         icm_rescan_work+42
> ```
> 
> This patch gates the `complete()` on an additional condition - that the
> Thunderbolt domain matches the domain of the device that was unplugged.
> 
> I only have a single system with multiple Thunderbolt domains - the Dell
> XPS9720. I am consistently able to reproduce the Oops on that system.
> Since running with this patch, I have not seen the Oops reoccur. I have
> also tested it on a Thinkpad X1-Gen12, and that system boots and runs
> well with it, but it only has a single Thunderbolt domain, so it does
> not exercise the multi-domain failure case.
> 
> Assisted by: LLM
> Signed-off-by: Jonathan Gopel <jgopel@gmail.com>

I dropped the Assisted-by per your previous email, did a couple of small
cosmetic changes to the patch, added Fixes and stable tags and applied to
thunderbolt.git/fixes, thanks!