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

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Message-ID: <12209.1416859494@warthog.procyon.org.uk>
Date:	Mon, 24 Nov 2014 20:04:54 +0000
From:	David Howells <dhowells@...hat.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	dhowells@...hat.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alexei Starovoitov <alexei.starovoitov@...il.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>,
	Paul McKenney <paulmck@...ux.vnet.ibm.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

Christian Borntraeger <borntraeger@...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
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