Re: rcu alignment warning tripping on m68k

[Greg Ungerer <gerg@uclinux.org>]

Hi Paul,

On 07/06/14 04:46, Paul E. McKenney wrote:
> 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 <gerg@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.

This change was prompted by this check tripping, so the alignment
issue is certainly real for m68k.

Regards
Greg


> (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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> 
>