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

[Eric Dumazet <dada1@cosmosbay.com>]

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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