Oleg Nesterov a écrit :
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.
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.
[*] : In particular, slab code managment does something special when
transfering local objects from local cpu A store to 'other cpus B'.
Other mechanisms should also use some kind of memory barrier in order to
transfer an object to another cpu too, or you could imagine in flight stores
from CPU A overwriting an object that was 'given' to CPU B.
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