linux-kernel - Re: [PATCH] tools/memory-model: remove rb-dep, smp_read_barrier_depends, and lockless

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Message-ID: <Pine.LNX.4.44L0.1802201005590.1428-100000@iolanthe.rowland.org>
Date:   Tue, 20 Feb 2018 10:11:06 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:     Peter Zijlstra <peterz@...radead.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Akira Yokosawa <akiyks@...il.com>,
        Kernel development list <linux-kernel@...r.kernel.org>,
        <mingo@...nel.org>, Will Deacon <will.deacon@....com>,
        <boqun.feng@...il.com>, <npiggin@...il.com>, <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [PATCH] tools/memory-model: remove rb-dep, smp_read_barrier_depends,
 and lockless_dereference

On Tue, 20 Feb 2018, Paul E. McKenney wrote:

> On Mon, Feb 19, 2018 at 09:28:44PM +0100, Peter Zijlstra wrote:
> > On Mon, Feb 19, 2018 at 11:41:23AM -0800, Paul E. McKenney wrote:
> > > On Mon, Feb 19, 2018 at 12:14:45PM -0500, Alan Stern wrote:
> > > > This leaves us with a question: Do we want to change the kernel by
> > > > adding memory barriers after unsuccessful RMW operations on Alpha, or
> > > > do we want to change the model by excluding such operations from
> > > > address dependencies?
> > > 
> > > I vote for adding the barrier on Alpha.  However, I don't know of any
> > > code in the Linux kernel that relies on read-to-read address dependency
> > > ordering headed by a failing RMW operation, so I don't feel all that
> > > strongly about this.
> > 
> > Right, but not knowing doesn't mean doesn't exist, and most certainly
> > doesn't mean will never exist.
> 
> Fair enough, safety first!
> 
> > > > Note that operations like atomic_add_unless() already include memory 
> > > > barriers.
> > > 
> > > And I don't see an atomic_add_unless_relaxed(), so we are good on this
> > > one.  So far, anyway!  ;-)
> > 
> > Not the point, add_unless() is a conditional operation, and therefore
> > doesn't need to imply anything when failing.
> 
> Plus it doesn't return a pointer, so there is no problem with dereferences.
> Unless someone wants to use its return value as an array index and rely
> on dependency ordering to the array, but I would NAK that use case.

You may not get the chance to NAK it.

We need to be consistent.  Array indexing is indeed a form of address
dependency, so either we assert that the dependency is enforced when
the array index is derived from a failed atomic operation, or else we
assert that failed atomic operations do not create address
dependencies.

Alan