linux-kernel - Re: [PATCH 2/2] tools/memory-model: Add write ordering by release-acquire and by locks

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Message-ID: <Pine.LNX.4.44L0.1807061629280.1396-100000@iolanthe.rowland.org>
Date:   Fri, 6 Jul 2018 16:37:21 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
cc:     Will Deacon <will.deacon@....com>,
        LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nicholas Piggin <npiggin@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Kernel development list <linux-kernel@...r.kernel.org>,
        <dlustig@...dia.com>
Subject: Re: [PATCH 2/2] tools/memory-model: Add write ordering by release-acquire
 and by locks

On Thu, 5 Jul 2018, Andrea Parri wrote:

> > At any rate, it looks like instead of strengthening the relation, I
> > should write a patch that removes it entirely.  I also will add new,
> > stronger relations for use with locking, essentially making spin_lock
> > and spin_unlock be RCsc.
> 
> Thank you.
> 
> Ah let me put this forward: please keep an eye on the (generic)
> 
>   queued_spin_lock()
>   queued_spin_unlock()
> 
> (just to point out an example). Their implementation (in part.,
> the fast-path) suggests that if we will stick to RCsc lock then
> we should also stick to RCsc acq. load from RMW and rel. store.

A very good point.  The implementation of those routines uses
atomic_cmpxchg_acquire() to acquire the lock.  Unless this is
implemented with an operation or fence that provides write-write
ordering (in conjunction with a suitable release), qspinlocks won't
have the ordering properties that we want.

I'm going to assume that the release operations used for unlocking
don't need to have any extra properties; only the lock-acquire
operations need to be special (i.e., stronger than a normal
smp_load_acquire). This suggests that atomic RMW functions with acquire
semantics should also use this stronger form of acquire.

Does anybody have a different suggestion?

Alan