linux-kernel - Re: [RFC][PATCH 4/4] futex: Rewrite FUTEX_UNLOCK

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Date:   Mon, 10 Oct 2016 13:40:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     mingo@...nel.org, juri.lelli@....com, rostedt@...dmis.org,
        xlpang@...hat.com, bigeasy@...utronix.de,
        linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
        jdesfossez@...icios.com, bristot@...hat.com
Subject: Re: [RFC][PATCH 4/4] futex: Rewrite FUTEX_UNLOCK_PI

On Mon, Oct 10, 2016 at 12:17:48PM +0200, Thomas Gleixner wrote:
> There is another problem with all that racing against fixup_owner()
> resp. fixup_pi_state_owner().
> 
> I fear, we need to rethink this whole locking/protection scheme from
> scratch.

So for pi_state (ie, the attach_to_pi_state() vs put_pi_state() race) I
can see two options, either we re-take hb->lock after we've completed
the futex_unlock_pi() in order to drop it, which is cringe worthy, or we
make pi_state RCU freed and replace that WARN_ON() in
attach_to_pi_state() with an atomic_inc_not_zero() and deal with the
fail case by going back to the caller and treating it like !top_waiter.

As to the rt_mutex vs futex state coherence, I think I can do all of
that with rt_mutex::wait_lock held, the alternative is doing part with
hb->lock and part with rt_mutex::wait_lock and parts with both, but
that's already hurting my head and I didn't even try yet.

Let me think a wee bit more on this..