Re: PATCH? rcu_do_batch: fix a pure theoretical memory ordering race

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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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