Re: [RFC PATCH 02/12] futex: Switch PI futex to use p->pi_futex_lock instead of p->pi_lock.

[K Prateek Nayak]

Hello Suleiman,

On 10/16/2026 11:29 AM, Suleiman Souhlal wrote:
>>> I suppose the alternatives would be to either figure out how to
>>> un-nest pi_lock from wait_lock in futex code or un-nesting the
>>> blocked_on lock from pi_lock in the scheduler.
>>
>> So the only time we try to grab pi_lock + blocked_lock is during
>> wakeup so I think doing that should be (theoretically) possible.
>>
>> The ttwu_runnable() handling is extremely painful unless we block
>> the delayed task fully. Since we have he new p->is_blocked state,
>> we can safely retain the p->blocked_on while running the task -
>> it is either cleared during unlock by previosu owner or will be
>> same as the lock on which the task blocked on.
>>
>>
>> Based on John's tree at commit 06ac43db4d8e ("[ANNOTATION] ===
>> Needs confirmation of functionality past this point ===") on
>> proxy-exec-v31-7.2-rc4 branch:
> 
> Sorry for the slow reply and thanks for the very quick patch.
> 
> Unfortunately, it doesn't seem like this approach works in the end,
> because there is still a pi_lock -> rq lock -> blocked_lock transitive
> relationship, which deadlocks when trying to get the pi_lock while
> holding the blocked_lock (when fixing up the pi_state owner in the
> futex code). :-(

Ah! So what you are saying is, since we have:

  p->pi_lock
    rq_lock /* ttwu_runnable() */

and

  rq_lock
   blocked_lock /* find_proxy_task() */

relations, doing a:

  blocked_lock
    p->pi_lock

would cause a deadlock via rq_lock. True that.

> 
> On the other hand, Peter's suggestion of moving the scheduler's use of
> pi_lock to a new lock, p->sched_lock, seems to work in my testing, but
> is kind of a scary change to me.
> 
> Maybe an acceptable alternative would be for the new futex to use a
> new lock (similarly to what the original patch does), but keep regular
> PI futexes using pi_lock, at least as long as they are still based on
> rtmutexes.

I guess that should do too, yes.

On a different note, why do we need the p->pi_lock/p->pi_futex_lock to
modify the ping_state->owner in ping_state_update_owner()? Isn't
ping_state->ping_mutex.wait_lock + owner->blocked_on locks enough?

Note: I'm still wrapping my head around the PING implementation so it
might take me a while to appreciate the need for the pi_futex_lock.
I'm using proxy-exec-v32-7.3-rc-tip-sched-core from John's tree at the
moment to jump around.

-- 
Thanks and Regards,
Prateek