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Date: Mon, 24 Nov 2014 20:07:03 +0100
From: Christian Borntraeger <borntraeger@de.ibm.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>
CC: David Howells <dhowells@redhat.com>,
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        Paolo Bonzini <pbonzini@redhat.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@kernel.org>,
        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
 types
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Am 24.11.2014 um 19:35 schrieb Linus Torvalds:
> On Mon, Nov 24, 2014 at 10:02 AM, Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
>>
>> If the goal is to catch non-scalar users, the following is shorter:
>> #define ACCESS_ONCE(x) (((typeof(x))0) + *(volatile typeof(x) *)&(x))
> 
> Me likey. It probably works well in practice, although I think
> 
>  - the "(typeof(x))0)" seems unnecessary and wrong. Why not just "0"?
> The typeof is not just longer, but it is incorrect for pointer types
> (you can add 0 to a pointer, but you cannot add two pointers together)
> 
>  - it does mean that the resulting type ends up being upgraded to
> "int", for the usual C type reasons.
> 
> Note that the "upgraded to 'int'" is true with or without the
> "(typeof(x))0". If you add two 'char' values, the addition is still
> done in 'int'.
> 
> Maybe you *meant* that typeof to fix the second problem, like so:
> 
>   (typeof(x)) (0 + *(volatile typeof(x) *)&(x))
> 
> Hmm? That casts the result of the addition, not the zero.

Looks really nice, but does not work with ACCESS_ONCE is on the left-hand side:


include/linux/rculist.h: In function 'hlist_add_before_rcu':
./arch/x86/include/asm/barrier.h:127:18: error: lvalue required as left operand of assignment
  ACCESS_ONCE(*p) = (v);      \

Alexei's variant is also broken:

include/linux/cgroup.h: In function 'task_css':
include/linux/compiler.h:381:40: error: invalid operands to binary + (have 'struct css_set *' and 'struct css_set * volatile')
 #define ACCESS_ONCE(x) (((typeof(x))0) + *(volatile typeof(x) *)&(x))

Anyone with a new propopal? ;-)                                        ^

Christian