linux-kernel - Re: [PATCH] compiler.h: cast away attributes in WRITE

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Message-ID: <20150804092027.GN25159@twins.programming.kicks-ass.net>
Date:	Tue, 4 Aug 2015 11:20:27 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	Andrey Konovalov <andreyknvl@...gle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] compiler.h: cast away attributes in WRITE_ONCE magic

On Tue, Aug 04, 2015 at 09:55:48AM +0200, Christian Borntraeger wrote:
> kernel build bot showed a warning triggered by commit
> 76695af20c01 ("locking, arch: use WRITE_ONCE()/READ_ONCE() in
> smp_store_release()/smp_load_acquire()"). Turns out that sparse
> does not like WRITE_ONCE accessing elements from the (sparse)
> rcu address space.
> 
> fs/afs/inode.c:448:9: sparse: incorrect type in initializer (different address spaces)
> fs/afs/inode.c:448:9:    expected struct afs_permits *__val
> fs/afs/inode.c:448:9:    got void [noderef] <asn:4>*<noident>
> 
> Solution is to force cast away the sparse attributes for the initializer
> of the union in WRITE_SAME. As this now gets too long, lets split

WRITE_ONCE, right?

> the macro.
> 
> Signed-off-by: Christian Borntraeger <borntraeger@...ibm.com>
> ---
>  include/linux/compiler.h | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index e08a6ae..c836eb2 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -252,7 +252,12 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
>  	({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
>  
>  #define WRITE_ONCE(x, val) \
> -	({ union { typeof(x) __val; char __c[1]; } __u = { .__val = (val) }; __write_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
> +({							\
> +	union { typeof(x) __val; char __c[1]; } __u =	\
> +		{ .__val = (__force typeof(x)) (val) }; \
> +	__write_once_size(&(x), __u.__c, sizeof(x));	\
> +	__u.__val;					\
> +})

Thanks!

READ_ONCE() doesn't have a similar problem because it only has the one
input type, which automagically becomes the output type, right?
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