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

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Message-ID: <20141124204614.GU5050@linux.vnet.ibm.com>
Date:	Mon, 24 Nov 2014 12:46:14 -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 12:19:11PM -0800, Linus Torvalds wrote:
> On Mon, Nov 24, 2014 at 11:42 AM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
> >
> > OK, how about the following?
> 
> Ugh. Disgusting.
> 
> Why the heck isn't it just "sizeof(*__vp) <= sizeof(long)"?
> 
> If the architecture has a 3-byte scalar type, then it probably has a
> 3-byte load.

Because I was allowing for the possibility of a 3-byte struct, which
as you point out below...

> > 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.
> 
> .. but that's *exactly* the gcc bug in question. It's a word-sized
> struct that gcc loads twice.

...was a stupid thought anyway.

OK, how about the attempt below?  The initialization of __p complains
for structures and unions, but gets optimized out.

							Thanx, Paul

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

#define get_scalar_volatile_pointer(x) ({ \
	typeof(x) __maybe_unused __p = 0; \
	volatile typeof(x) *__vp = &(x); \
	BUILD_BUG_ON(sizeof(*__vp) > sizeof(long)); \
	__vp; })
#define ACCESS_ONCE(x) (*get_scalar_volatile_pointer(x))

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