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

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Date:	Mon, 24 Nov 2014 12:48:05 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:	Christian Borntraeger <borntraeger@...ibm.com>,
	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>,
	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 12:29:07PM -0800, Alexei Starovoitov wrote:
> 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))

You beat me to it.  ;-)

However, your approach would allow an ACCESS_ONCE() of a long long
to compile on 32-bit systems where it has to be broken up into a
pair of 32-bit accesses.

							Thanx, Paul

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