linux-kernel - Re: [RFC patch] spindep: add cross cache lines checking

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Date:	Wed, 07 Mar 2012 16:23:27 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Arnd Bergmann <arnd@...db.de>, gcc@....gnu.org
Cc:	Ingo Molnar <mingo@...e.hu>, tglx@...utronix.de,
	"mingo@...hat.com" <mingo@...hat.com>, hpa@...or.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	x86@...nel.org, andi.kleen@...el.com, gcc-help@....gnu.org
Subject: Re: [RFC patch] spindep: add cross cache lines checking

On Tue, 2012-03-06 at 09:32 +0000, Arnd Bergmann wrote:
> On Tuesday 06 March 2012, Alex Shi wrote:
> > I have one concern and one questions here:
> > concern: maybe the lock is in a well designed 'packed' struct, and it is
> > safe for cross lines issue. but __alignof__ will return 1;
> > 
> > struct abc{
> >         raw_spinlock_t lock1;
> >         char            a;
> >         char            b;
> > }__attribute__((packed));
> > 
> > Since the lock is the first object of struct, usually it is well placed.
> 
> No, it's actually not. The structure has an external alignment of 1, so
> if you have an array of these or put it into another struct like
> 
> struct xyz {
> 	char x;
> 	struct abc a;
> };
> 
> then it will be misaligned. Thre is no such thing as a well designed 'packed'
> struct. The only reason to use packing is to describe structures we have no
> control over such as hardware layouts and on-wire formats that have unusal
> alignments, and those will never have a spinlock on them.

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
 
diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index 7df6c17..df8a992 100644
--- a/include/linux/spinlock.h
+++ b/include/linux/spinlock.h
@@ -96,11 +96,13 @@
 do {								\
 	static struct lock_class_key __key;			\
 								\
+	BUILD_BUG_ON(__alignof__(lock) == 1);			\
 	__raw_spin_lock_init((lock), #lock, &__key);		\
 } while (0)
 
 #else
 # define raw_spin_lock_init(lock)				\
+	BUILD_BUG_ON(__alignof__(lock) == 1);			\
 	do { *(lock) = __RAW_SPIN_LOCK_UNLOCKED(lock); } while (0)
 #endif
 
===

Btw, 
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. 

 
> 
> 	Arnd


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