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

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Message-ID: <CAADnVQJQydX9OU_rem+BObR0eWc-jrrwirUYVKH9rnN=Z8LG6A@mail.gmail.com>
Date:	Mon, 24 Nov 2014 10:02:52 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Christian Borntraeger <borntraeger@...ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org, linux-mips@...ux-mips.org,
	linux-x86_64@...r.kernel.org, linux-s390@...r.kernel.org,
	Paolo Bonzini <pbonzini@...hat.com>,
	paulmck@...ux.vnet.ibm.com, Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.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 5:30 AM, David Howells <dhowells@...hat.com> wrote:
> Christian Borntraeger <borntraeger@...ibm.com> wrote:
>
>> +#define get_scalar_volatile_pointer(x) ({ \
>> +     typeof(x) *__p = &(x); \
>> +     volatile typeof(x) *__vp = __p; \
>> +     (void)(long)*__p; __vp; })
>> +#define ACCESS_ONCE(x) (*get_scalar_volatile_pointer(x))

If the goal is to catch non-scalar users, the following is shorter:
#define ACCESS_ONCE(x) (((typeof(x))0) + *(volatile typeof(x) *)&(x))

it will block union and struct accesses.
If you want to allow union and disallow struct, then replace + with ,
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