From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S933636AbcKXQw4 (ORCPT <rfc822;w@1wt.eu>);
        Thu, 24 Nov 2016 11:52:56 -0500
Received: from bombadil.infradead.org ([198.137.202.9]:35975 "EHLO
        bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S932584AbcKXQwy (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 24 Nov 2016 11:52:54 -0500
Date: Thu, 24 Nov 2016 17:52:41 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: mingo@kernel.org, juri.lelli@arm.com, rostedt@goodmis.org,
        xlpang@redhat.com, bigeasy@linutronix.de, linux-kernel@vger.kernel.org,
        mathieu.desnoyers@efficios.com, jdesfossez@efficios.com,
        bristot@redhat.com
Subject: Re: [RFC][PATCH 4/4] futex: Rewrite FUTEX_UNLOCK_PI
Message-ID: <20161124165241.GF3174@twins.programming.kicks-ass.net>
References: <20161003091234.879763059@infradead.org>
 <20161003091847.704255067@infradead.org>
 <20161007112143.GJ3117@twins.programming.kicks-ass.net>
 <alpine.DEB.2.20.1610081726270.5222@nanos>
 <20161008165540.GI3568@worktop.programming.kicks-ass.net>
 <alpine.DEB.2.20.1610101215040.7166@nanos>
 <20161021122735.GA3117@twins.programming.kicks-ass.net>
 <alpine.DEB.2.20.1610272052580.4913@nanos>
 <20161123192005.GA3107@twins.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20161123192005.GA3107@twins.programming.kicks-ass.net>
User-Agent: Mutt/1.5.23.1 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, Nov 23, 2016 at 08:20:05PM +0100, Peter Zijlstra wrote:
> > +	if (oldowner == &init_task && uval != 0) {
> > +		raw_spin_lock(&pi_state->owner->pi_lock);
> > +		list_del_init(&pi_state->list);
> > +		raw_spin_unlock(&pi_state->owner->pi_lock);
> > +		pi_state->owner = NULL;
> > +		return -EAGAIN;

> > @@ -2679,6 +2690,10 @@ static int futex_lock_pi(u32 __user *uad
> >  
> >  out_put_key:
> >  	put_futex_key(&q.key);
> > +
> > +	if (ret == -EAGAIN)
> > +		goto retry;
> > +
> 
> And this is far too clever and really needs a comment. So the crucial
> point is that this is after unqueue_me_pi(), which drops the pi_state
> and loops back to lookup the pi_state again, which, hopefully, has now
> been completely destroyed -- and therefore we hit the regular
> attach_to_pi_owner() path, fixing up our 'funny' state.
> 

I'm stumped on REQUEUE_PI.. this relies on attach_to_pi_owner() and
fixup_owner() being in the same function. But this is not the case for
requeue. WAIT_REQUEUE has the fixup, as its return path finds it has
acquired the outer pi-futex (uaddr2), but the lookup_pi_state() stuff is
done by CMP_REQUEUE, which does the actual transfer of the waiters from
inner futex (uaddr1) to outer futex (uaddr2).


Maybe I can restructure things a bit, I think CMP_REQUEUE would also
know who actually acquired the outer-futex, but I have to think more on
this and the brain is pretty fried...