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

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Date:   Tue, 24 Oct 2017 11:34:13 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        "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()


* 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.

Thanks,

	Ingo