Re: 6.18.13 iwlwifi deadlock allocating cma while work-item is active.

[Ben Greear <greearb@candelatech.com>]

On 3/2/26 00:07, Johannes Berg wrote:
> On Sun, 2026-03-01 at 07:38 -0800, Ben Greear wrote:
>> On 2/27/26 08:31, Ben Greear wrote:
>>> On 2/23/26 14:36, Ben Greear wrote:
>>>> Hello,
>>>>
>>>> I hit a deadlock related to CMA mem allocation attempting to flush all work
>>>> while holding some wifi related mutex, and with a work-queue attempting to process a wifi regdomain
>>>> work item.  I really don't see any good way to fix this,
>>>> it would seem that any code that was holding a mutex that could block a work-queue
>>>> cannot safely allocate CMA memory?  Hopefully someone else has a better idea.
>>>
>>> I tried using a kthread to do the regulatory domain processing instead of worker item,
>>> and that seems to have solved the problem.  If that seems reasonable approach to
>>> wifi stack folks, I can post a patch.
>>
>> The other net/wireless work-item 'disconnect_work' also needs to be moved to the kthread
>> for the same reason....
> 
> I don't think we want to use a kthread for this, it doesn't really make
> sense.
> 
> Was this with lockdep? If so, it complain about anything?
> 
> I'm having a hard time seeing why it would deadlock at all when wifi
> uses  schedule_work() and therefore the system_percpu_wq, and
> __lru_add_drain_all() flushes lru_add_drain_work on mm_percpu_wq, and
> lru_add_and_bh_lrus_drain() doesn't really _seem_ to do anything related
> to RTNL etc.?
> 
> I think we need a real explanation here rather than "if I randomly
> change this, it no longer appears".

The path where iwlwifi acquires CMA holds rtnl and/or wiphy locks before
allocating CMA memory, as expected.

And the CMA allocation path attempts to flush the work queues in
at least some cases.

If there is a work item queued that is trying to grab rtnl and/or wiphy lock
when CMA attempts to flush, then the flush work cannot complete, so it deadlocks.

Lockdep doesn't warn about this.

Thanks,
Ben

-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com