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Date: Mon, 24 Nov 2014 12:48:05 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        David Howells <dhowells@redhat.com>,
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        Paolo Bonzini <pbonzini@redhat.com>, Ingo Molnar <mingo@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH/RFC 7/7] kernel: Force ACCESS_ONCE to work only on scalar
 types
Message-ID: <20141124204805.GV5050@linux.vnet.ibm.com>
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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@de.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