Re: [RFC PATCH 07/12] futex: Make FUTEX_*_PING use Proxy Execution.

[K Prateek Nayak]

On 9/17/2026 9:06 PM, Peter Zijlstra wrote:
>>> Could we handle cycles in find_proxy_task(), or add a chain walk for
>>> deadlock detection for FUTEX_LOCK_PING like rtmutex?
>>
>> https://lore.kernel.org/lkml/20260714152220.4046736-1-soolaugust@xxxxxxxxx/
> 
> Ah yes, that thing. I would suggest to still have a hard-coded limit,
> but perhaps in addition to the sequence mark.
> 
> Without a hard-coded limit, userspace is free to create chains of
> arbitrary length. This should be discouraged :-)
> 
> Also, we need to be able to return -EDEADLK to userspace.
>
> Ideally userspace gets to have an extra graph walk on block though, and
> not rely on pick time sanity checks.

Ack!

One way to do that is by setting the "p->blocked_on" to -1 when
find_proxy_task() detects a chain and then clear the "p->is_blocked"
making it runnable.

Once the task exits out of schedule_preempt_disabled() and grabs the
wait_lock, we can check the "p->blocked_on" to return -EDEADLK / stop
proxy and fully block the task.

Would that be acceptable?

-- 
Thanks and Regards,
Prateek