linux-kernel - Re: [PATCH 09/19] net: average: Kill off ACCESS

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Message-Id: <20171024134939.GY3659@linux.vnet.ibm.com>
Date:   Tue, 24 Oct 2017 06:49:39 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Johannes Berg <johannes@...solutions.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "will.deacon@....com" <will.deacon@....com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "snitzer@...hat.com" <snitzer@...hat.com>,
        "thor.thayer@...ux.intel.com" <thor.thayer@...ux.intel.com>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "tj@...nel.org" <tj@...nel.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 09/19] net: average: Kill off ACCESS_ONCE()

On Tue, Oct 24, 2017 at 11:34:13AM +0200, Ingo Molnar wrote:
> 
> * Johannes Berg <johannes@...solutions.net> wrote:
> 
> > On Mon, 2017-10-23 at 21:07 +0000, Paul E. McKenney wrote:
> > > From: Mark Rutland <mark.rutland@....com>
> > > 
> > > For several reasons, it is desirable to use {READ,WRITE}_ONCE() in
> > > preference to ACCESS_ONCE(), and new code is expected to use one of the
> > > former. So far, there's been no reason to change most existing uses of
> > > ACCESS_ONCE(), as these aren't currently harmful.
> > > 
> > > However, for some features it is necessary to instrument reads and
> > > writes separately, which is not possible with ACCESS_ONCE(). This
> > > distinction is critical to correct operation.
> > > 
> > > It's possible to transform the bulk of kernel code using the Coccinelle
> > > script below. However, this doesn't pick up some uses, including those
> > > in <linux/average.h>. As a preparatory step, this patch converts the
> > > file to use {READ,WRITE}_ONCE() consistently.
> > > 
> > > At the same time, this patch addds missing includes necessary for
> > > {READ,WRITE}_ONCE(), *BUG_ON*(), and ilog2().
> > > 
> > > ----
> > > virtual patch
> > > 
> > > @ depends on patch @
> > > expression E1, E2;
> > > @@
> > > 
> > > - ACCESS_ONCE(E1) = E2
> > > + WRITE_ONCE(E1, E2)
> > > 
> > > @ depends on patch @
> > > expression E;
> > > @@
> > > 
> > > - ACCESS_ONCE(E)
> > > + READ_ONCE(E)
> > > ----
> > > 
> > > Signed-off-by: Mark Rutland <mark.rutland@....com>
> > > Cc: Johannes Berg <johannes.berg@...el.com>
> > 
> > Reviewed-by: Johannes Berg <johannes@...solutions.net>
> 
> Thanks!
> 
> > Let me know if you want me to apply this, since I seem to be the
> > average.h maintainer :-)
> 
> Would be nice to keep these patches together if possible, so that we can remove 
> ACCESS_ONCE() - or at least add a build warning.

My goal is to remove it.  After all, I am the guy who added it.  ;-)

							Thanx, Paul