[CCd to linux-kernel]
Martijn van Oosterhout <kleptog@cupid.suninternet.com> wrote:
> It seems indeed that the module and the kernel were
> compiled with different versions. The kernel with 2.95.2
> (Debian Potato) on a dual-PII and the module on the
> actually machine with 2.7.2.3 (Debian Slink).
Exactly my config too.
> I checked a few header files to see if there was anything
> obvious. All I could come up with was in the
> <linux/netdevice.h> header, in the declaration for
> struct hh_cache. The member named hh_type it in a
> position to make the alignment ambiguous. This would
> explain the 2 byte shift.
Good spot. I've always suspected that the hh_cache was involved in
some way. This struct is (in 2.2.15pre7 <linux/netdevice.h>) as
follows, and it's even worse than seen from your first look:
struct hh_cache
{
struct hh_cache *hh_next; /* Next entry */
atomic_t hh_refcnt; /* number of users */
unsigned short hh_type; /* protocol identifier, f.e ETH_P_IP */
/*** Possible alignment problem here ***/
int (*hh_output)(struct sk_buff *skb);
rwlock_t hh_lock;
/*** And another possible alignment problem here ***/
/* cached hardware header; allow for machine alignment needs. */
unsigned long hh_data[16/sizeof(unsigned long)];
};
As I've already mentioned I've got a _four_ byte shift. The reason is
probably the rwlock_t definition in <asm-i386/spinlock.h> for the UP case:
#if (__GNUC__ > 2) || (__GNUC__ == 2 && __GNUC_MINOR__ >= 8)
typedef struct { } rwlock_t;
#define RW_LOCK_UNLOCKED (rwlock_t) { }
#else
typedef struct { int gcc_is_buggy; } rwlock_t;
#define RW_LOCK_UNLOCKED (rwlock_t) { 0 }
#endif
This is not only an alignment problem but an explicit compiler
dependency as well :-(
Alan (or whoever is responsible :-), is there any chance to get this
fixed in some way?
> Suggestion: use attribute((packed)), make it an int
> and move that member elsewhere.
Packing doesn't look right to me, as misaligning the array and the
lock (for SMP) is performance-unfriendly. Reordering could help, like
this:
struct hh_cache
{
struct hh_cache *hh_next; /* Next entry */
atomic_t hh_refcnt; /* number of users */
int (*hh_output)(struct sk_buff *skb);
/* cached hardware header; allow for machine alignment needs. */
unsigned long hh_data[16/sizeof(unsigned long)];
rwlock_t hh_lock;
unsigned short hh_type; /* protocol identifier, f.e ETH_P_IP */
};
Olaf
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This archive was generated by hypermail 2b29 : Thu Mar 23 2000 - 21:00:20 EST