On Fri, May 30, 2014 at 11:29:41AM +1000, Greg Ungerer wrote:
> On 29/05/14 23:11, One Thousand Gnomes wrote:
> > On Thu, 29 May 2014 12:08:32 +1000
> > Greg Ungerer <g...@uclinux.org> wrote:
> >
> >> Hi All,
> >>
> >> Inside kernel/rcy/tree.c in __call_rcu() it does an alignment check on
> >> the head pointer passed in. This trips on m68k systems, because they only
> >> need alignment of 32bit quantities to 16bit boundaries.
> >
> > __alignof perhaps ?
>
> That might do. Change then becomes something like:
>
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2467,7 +2467,7 @@ __call_rcu(struct rcu_head *head, void (*func)(struct
> rcu_
> unsigned long flags;
> struct rcu_data *rdp;
>
> - WARN_ON_ONCE((unsigned long)head & 0x3); /* Misaligned rcu_head! */
> + WARN_ON_ONCE((unsigned long)head & (__alignof__(head) - 1)); /*
> Misaligned rcu_head! */
Hmmm... The purpose of the check is to reserve the low-order bits to
allow RCU to classify callbacks as being time-critical or not. RCU
can probably live with a single bit, but if there is some architecture
out there that simply refuses to do alignment, I need to know about it.
(See "git show 0bb7b59d6e2b8" for more info.)
So how about this instead?
- WARN_ON_ONCE((unsigned long)head & 0x1); /* Misaligned rcu_head! */
(Trying to remember if I have seen Linux kernel code that uses both
the lower bits...)
Thanx, Paul
> if (debug_rcu_head_queue(head)) {
> /* Probable double call_rcu(), so leak the callback. */
> ACCESS_ONCE(head->func) = rcu_leak_callback;
>
> Thanks
> Greg
>
>
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