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Date: Mon, 24 Nov 2014 11:42:00 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
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        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
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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@de.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))