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

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Message-ID: <alpine.DEB.2.20.1610081903550.5222@nanos>
Date:   Sat, 8 Oct 2016 19:06:04 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
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 Sat, 8 Oct 2016, Peter Zijlstra wrote:
> > So in this case we tell the caller on CPU 1 that the futex is in
> > inconsistent state, because pistate->owner still points to the unlocking
> > task while the user space value alread shows the new owner. So this sanity
> > check triggers and we simply fail while we should not. It's [10] in the
> > state matrix above attach_to_pi_state().
> 
> Urgh, yes. I think I can cure that, by taking
> pi_state->pi_mutex.wait_lock in attach_to_pi_state(), but blergh.
> 
> > I suspect that there are more issues like this, especially since I did not
> > look at requeue_pi yet, but by now my brain is completely fried.
> 
> Yes, I know about fried brains :-( This stuff has far too many moving
> parts. I've been staring at this stuff far too long.
> 
> Also, we need better tools to stress this stuff.

I tried to come up with something which forces all corner cases of this
years ago and failed. The state space seems to be infinite.

Thanks,

	tglx