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
