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From: David Howells <dhowells@redhat.com>
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References: <547381D7.2070404@de.ibm.com> <1416834210-61738-1-git-send-email-borntraeger@de.ibm.com> <1416834210-61738-8-git-send-email-borntraeger@de.ibm.com> <15567.1416835858@warthog.procyon.org.uk> <CAADnVQJQydX9OU_rem+BObR0eWc-jrrwirUYVKH9rnN=Z8LG6A@mail.gmail.com> <CA+55aFxc72VsGTw4yFdeC1Sq65RUjYLKPD1ORnXB2d18WBMzvg@mail.gmail.com>
To: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: dhowells@redhat.com, Linus Torvalds <torvalds@linux-foundation.org>,
        Alexei Starovoitov <alexei.starovoitov@gmail.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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Date: Mon, 24 Nov 2014 20:04:54 +0000
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Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> 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? ;-)                                        ^

Reserve ACCESS_ONCE() for reading and add an ASSIGN_ONCE() or something like
that for writing?

David