Re: [RFC, PATCH] kernel/rcu: add kfree_rcu

[Manfred Spraul <manfred@colorfullife.com>]

Paul E. McKenney wrote:
> I would suggest instead using the bottom bit to differentiate between
> these two cases, especially given that your approach makes it impossible
> for callback processing to notice a NULL function pointer.  In addition,
> this approach would allow different types of allocators to be specified
> should this later prove to be helpful.  You should not have to shift the
> offset because the rcu_head offset should always be a multiple of four
> (or eight on 64-bit architectures).
>   
We must be careful: rcu_head might be always aligned, but are function 
pointers always aligned?
The x86 hardware allows arbitrary function pointers, I'm not sure what 
gcc would do if '--falign-functions=0' is used.
Are there other codepaths that assume that the lowest bit of a function 
pointer is never set?

> And we really are running into bugs that are detected by RCU's seeing a
> null function pointer in the rcu_head structure at callback-invocation
> time.  So, whatever encoding you choose, please leave a function-pointer
> value of zero as an invalid value!
>   
Ok.

>> --- a/kernel/rcutree.c
>> +++ b/kernel/rcutree.c
>> @@ -901,7 +901,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
>>  	while (list) {
>>  		next = list->next;
>>  		prefetch(next);
>> -		list->func(list);
>> +		rcu_docallback(list);
>>     
>
> Good, you got all three of them!  ;-)
>
>   
The patch was tested against rcutree ;-)

--
    Manfred