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SubjectRe: PATCH? rcu_do_batch: fix a pure theoretical memory ordering race
Oleg Nesterov a écrit :
> On 12/03, Eric Dumazet wrote:
>> Oleg Nesterov a ?crit :
>>> On top of rcu-add-a-prefetch-in-rcu_do_batch.patch
>>>
>>> rcu_do_batch:
>>>
>>> struct rcu_head *next, *list;
>>>
>>> while (list) {
>>> next = list->next; <------ [1]
>>> list->func(list);
>>> list = next;
>>> }
>>>
>>> We can't trust *list after list->func() call, that is why we load
>>> list->next
>>> beforehand. However I suspect in theory this is not enough, suppose that
>>>
>>> - [1] is stalled
>>>
>>> - list->func() marks *list as unused in some way
>>>
>>> - another CPU re-uses this rcu_head and dirties it
>>>
>>> - [1] completes and gets a wrong result
>>>
>>> This means we need a barrier in between. mb() looks more suitable, but I
>>> think
>>> rmb() should suffice.
>>>
>> Well, hopefully the "list->func()" MUST do the right thing [*], so your
>> patch is not necessary.
>
> Yes, I don't claim it is necessary, note the "pure theoretical".
>
>> For example, most structures are freed with kfree()/kmem_cache_free() and
>> these functions MUST imply an smp_mb() [if/when exchanging data with other
>> cpus], or else many uses in the kernel should be corrected as well.
>
> Yes, mb() is enough (wmb() isn't) and kfree()/kmem_cache_free() are ok.
> And I don't know any example of "unsafe" code in that sense.
>
> However I believe it is easy to make the code which is correct from the
> RCU's API pov, but unsafe.

Yes, but how is it related to RCU ?
I mean, rcu_do_batch() is just a loop like others in kernel.
The loop itself is not buggy, but can call a buggy function, you are right.
A smp_rmb() wont avoid all possible bugs...

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