linux-kernel - [PATCH 0/1] futex: don't spin waiting for PF_EXITING -> PF

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Date:	Mon, 16 Feb 2015 21:13:13 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Darren Hart <darren@...art.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	Jerome Marchand <jmarchan@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Mateusz Guzik <mguzik@...hat.com>,
	linux-kernel@...r.kernel.org,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	Pavel Emelyanov <xemul@...allels.com>
Subject: [PATCH 0/1] futex: don't spin waiting for PF_EXITING ->
	PF_EXITPIDONE transition

On 02/02, Oleg Nesterov wrote:
>
> And another question. Lets forget about this ->mm check. I simply can not
> understand this
>
>	ret = (p->flags & PF_EXITPIDONE) ? -ESRCH : -EAGAIN
>
> logic in attach_to_pi_owner(). First of all, why do we need to retry if
> PF_EXITING is set but PF_EXITPIDONE is not? Why we can not simply ignore
> PF_EXITING and rely on exit_pi_state_list() if PF_EXITPIDONE is not set?
>
> I must have missed something but this looks buggy, I do not see any
> preemption point
  ^^^^^^^^^^

I meant synchronization point, sorry for confusion.

> in this "retry" loop. Suppose that max_cpus=1 and rt_task()
> preempts the non-rt PF_EXITING owner. Looks like futex_lock_pi() can spin
> forever in this case? (OK, ignoring RT throttling).

Finally I forced myself to try to make the 1st patch ;) To remind, we have
more problems with robust+pi futexes, this needs another patch(es). Otherwise
we could (probably) even kill PF_EXITPIDONE.

Peter. I have no idea how to test it (except it obviously fixes the test-
case I sent before). IOW: please review.

And I still fail to understand why this PF_EXITING logic was added in the
first place. So I also have the problem with the changelog, it merely says
"don't do this because this is not needed".

On top of "check PF_KTHREAD rather than !p->mm to filter out kthreads" but
doesn't depend on it.

Oleg.

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