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From: Arnd Bergmann <arnd@arndb.de>
To: Alex Shi <alex.shi@intel.com>
Subject: Re: [RFC patch] spindep: add cross cache lines checking
Date: Wed, 7 Mar 2012 11:54:35 +0000
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On Wednesday 07 March 2012, Alex Shi wrote:

> Understand. thx. So is the following checking that your wanted?
> ===
> diff --git a/include/linux/rwlock.h b/include/linux/rwlock.h
> index bc2994e..64828a3 100644
> --- a/include/linux/rwlock.h
> +++ b/include/linux/rwlock.h
> @@ -21,10 +21,12 @@
>  do {								\
>  	static struct lock_class_key __key;			\
>  								\
> +	BUILD_BUG_ON(__alignof__(lock) == 1);			\
>  	__rwlock_init((lock), #lock, &__key);			\
>  } while (0)
>  #else
>  # define rwlock_init(lock)					\
> +	BUILD_BUG_ON(__alignof__(lock) == 1);			\
>  	do { *(lock) = __RW_LOCK_UNLOCKED(lock); } while (0)
>  #endif

I think the check should be (__alignof__(lock) < __alignof__(rwlock_t)),
otherwise it will still pass when you have structure with attribute((packed,aligned(2)))

> 1, it is alignof bug for default gcc on my fc15 and Ubuntu 11.10 etc?
> 
> struct sub {
>         int  raw_lock;
>         char a;
> };
> struct foo {
>         struct sub z;
>         int slk;
>         char y;
> }__attribute__((packed));
> 
> struct foo f1;
> 
> __alignof__(f1.z.raw_lock) is 4, but its address actually can align on
> one byte. 

That looks like correct behavior, because the alignment of raw_lock inside of
struct sub is still 4. But it does mean that there can be cases where the
compile-time check is not sufficient, so we might want the run-time check
as well, at least under some config option.

	Arnd