linux-kernel - Re: [PATCH/RFC 7/7] kernel: Force ACCESS

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Message-ID: <CAADnVQJ6eXGiasoQwyAzuejLncEHdy6MOf+m3AHnpjgn0h3+OQ@mail.gmail.com>
Date:	Mon, 24 Nov 2014 12:29:07 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	linux-mips <linux-mips@...ux-mips.org>,
	linux-x86_64@...r.kernel.org,
	linux-s390 <linux-s390@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>
Subject: Re: [PATCH/RFC 7/7] kernel: Force ACCESS_ONCE to work only on scalar types

On Mon, Nov 24, 2014 at 11:07 AM, Christian Borntraeger
<borntraeger@...ibm.com> wrote:
>
> Anyone with a new propopal? ;-)                                        ^

one more proposal :)
#define __ACCESS_ONCE(x) ({ typeof(x) __var = 0; (volatile typeof(x) *)&(x); })
#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))

works as lvalue...
the basic idea is the same:
constant zero can be used to initialize any scalar
(including pointers), but unions and structs will fail to compile as:
"error: invalid initializer"

If I'm reading pr58145 gcc bug report correctly, it
miscompiles only structs, so we can let ACCESS_ONCE
to work on unions. Then the following will rejects structs only:
#define __ACCESS_ONCE(x) ({ (typeof(x))0; (volatile typeof(x) *)&(x); })
#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))
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