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Re: [RFC patch] spindep: add cross cache lines checking
- From: Arnd Bergmann <arnd at arndb dot de>
- To: Alex Shi <alex dot shi at intel dot com>
- Cc: gcc at gcc dot gnu dot org, Ingo Molnar <mingo at elte dot hu>, tglx at linutronix dot de, "mingo at redhat dot com" <mingo at redhat dot com>, hpa at zytor dot com, akpm at linux-foundation dot org, linux-kernel at vger dot kernel dot org, x86 at kernel dot org, andi dot kleen at intel dot com, gcc-help at gcc dot gnu dot org
- Date: Wed, 7 Mar 2012 11:54:35 +0000
- Subject: Re: [RFC patch] spindep: add cross cache lines checking
- References: <1330917630.18835.44.camel@debian> <201203060932.45223.arnd@arndb.de> <1331108607.18835.343.camel@debian>
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