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

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Date:	Mon, 24 Nov 2014 11:42:00 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Christian Borntraeger <borntraeger@...ibm.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	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 11:14:42AM -0800, Linus Torvalds wrote:
> On Mon, Nov 24, 2014 at 11:07 AM, Christian Borntraeger
> <borntraeger@...ibm.com> wrote:
> >
> > Looks really nice, but does not work with ACCESS_ONCE is on the left-hand side:
> 
> Oh, I forgot about that. And that was indeed why I had done that whole
> helper macro originally, with ACCESS_ONCE() itself just being the
> dereference of the pointer.

OK, how about the following?

It complains if the variable is too large, for example, long long on
32-bit systems or large structures.  It is OK loading from and storing
to small structures as well, which I am having a hard time thinking of
as a disadvantage.

							Thanx, Paul

------------------------------------------------------------------------

#define get_scalar_volatile_pointer(x) ({ \
	volatile typeof(x) *__vp = &(x); \
	BUILD_BUG_ON(sizeof(*__vp) != sizeof(char) && \
		     sizeof(*__vp) != sizeof(short) && \
		     sizeof(*__vp) != sizeof(int) && \
		     sizeof(*__vp) != sizeof(long)); \
	__vp; })
#define ACCESS_ONCE(x) (*get_scalar_volatile_pointer(x))

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