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 S933088AbeBVPvs (ORCPT <rfc822;w@1wt.eu>);
        Thu, 22 Feb 2018 10:51:48 -0500
Received: from merlin.infradead.org ([205.233.59.134]:54796 "EHLO
        merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S933002AbeBVPvr (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 22 Feb 2018 10:51:47 -0500
Date: Thu, 22 Feb 2018 16:51:43 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Andrea Parri <parri.andrea@gmail.com>
Subject: Re: [RFC tip/locking/lockdep v5 05/17] lockdep: Extend __bfs() to
 work with multiple kinds of dependencies
Message-ID: <20180222155143.GV25235@hirez.programming.kicks-ass.net>
References: <20180222070904.548-1-boqun.feng@gmail.com>
 <20180222070904.548-6-boqun.feng@gmail.com>
 <20180222142614.GR25201@hirez.programming.kicks-ass.net>
 <20180222151210.jwxjchywk4jfecyf@tardis>
 <20180222153034.GO25181@hirez.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20180222153034.GO25181@hirez.programming.kicks-ass.net>
User-Agent: Mutt/1.9.2 (2017-12-15)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Thu, Feb 22, 2018 at 04:30:34PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 22, 2018 at 11:12:10PM +0800, Boqun Feng wrote:
> > On Thu, Feb 22, 2018 at 03:26:14PM +0100, Peter Zijlstra wrote:
> 
> > > However, I would suggest:
> > > 
> > > static inline bool is_xr(u16 dep)
> > > {
> > > 	return !!(dep & (DEP_NR_MASK | DEP_RR_MASK));
> > > }
> > > 
> > > static inline bool is_rx(u16 dep)
> > > {
> > > 	return !!(dep & (DEP_RN_MASK | DEP_RR_MASK));
> > > }
> > > 
> > > 		/* Skip *R -> R* relations */
> > > 		if (have_xr && is_rx(entry->dep))
> > > 			continue;
> > 
> > I don't think this works, if we pick a *R for previous entry, and for
> > current entry, we have RR, NN and NR, your approach will skip the
> > current entry, but actually we can pick NN or NR (of course, in __bfs(),
> > we can greedily pick NN, because if NR causes a deadlock, so does NN).
> 
> I don't get it, afaict my suggestion is identical.
> 
> You skip condition: pick_dep() < 0, evaluates to:
> 
> 	is_rr && (!NN_MASK && !NR_MASK) :=
> 	is_rr && (RN_MASK | RR_MASK)
> 
> Which is exactly what I have.

Ooh, I think I see what I did wrong, would something like:

	if (have_xr && !is_nx(entry-dep))

work? That's a lot harder to argue about though, still much better than
that tri-state pick thing.

> If that is satisfied, you set entry->is_rr to pick_dep(), which his
> harder to evaluate, but is something like:
> 
> 	is_rr && NR_MASK || !(NN_MASK | RN_MASK) :=
> 	is_rr && NR_MASK || (NR_MASK | RR_MASK) :=
> 	(NR_MASK | RR_MASK)
> 
> (because is_rr && RR_MASK will have been skipped)
> 
> > > 
> > > 		entry->have_xr = is_xr(entry->dep);

This one I think is still correct though.