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SubjectRe: [RFC patch] spindep: add cross cache lines checking
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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


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