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Date: Mon, 24 Nov 2014 21:28:05 +0100
From: Christian Borntraeger <borntraeger@de.ibm.com>
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To: paulmck@linux.vnet.ibm.com, Linus Torvalds <torvalds@linux-foundation.org>
CC: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
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        Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH/RFC 7/7] kernel: Force ACCESS_ONCE to work only on scalar
 types
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Am 24.11.2014 um 20:42 schrieb Paul E. McKenney:
> 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.

Well, the motivation for this series was that gcc  4.6 and 4.7 might ignore volatile for
such a case, see the original thread and this data structure

union ipte_control {
        unsigned long val;
        struct {
                unsigned long k  : 1;
                unsigned long kh : 31;
                unsigned long kg : 32;
        };
};

> ------------------------------------------------------------------------
> 
> #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))
> 

This gives also several compiler errors when accessing u64 on a 32bit system. This is expected, but more widespread than expected - ouch.

Christian