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

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Message-ID: <15567.1416835858@warthog.procyon.org.uk>
Date:	Mon, 24 Nov 2014 13:30:58 +0000
From:	David Howells <dhowells@...hat.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	dhowells@...hat.com, 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, mingo@...nel.org,
	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

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))

Might this cause two loads from memory under some conditions?  Once for the
fourth line and once for the fifth?

(Apologies if this has already been discussed)

David
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