Jesper Dangaard Brouer wrote:
On Wed, 2009-06-24 at 15:58 +0200, Patrick McHardy wrote:
Jesper Dangaard Brouer wrote:
Adjusting SLAB_DESTROY_BY_RCU flags.
kmem_cache_create("nf_conntrack", ...) does not need the
SLAB_DESTROY_BY_RCU flag.
It does need it. We're using it instead of call_rcu() for conntracks.
But the
kmem_cache_create("nf_conntrack_expect", ...) should use the
SLAB_DESTROY_BY_RCU flag, because it uses a call_rcu() callback to
invoke kmem_cache_free().
No, using call_rcu() means we don't need SLAB_DESTROY_BY_RCU.
Please see the note in include/linux/slab.h.
Oh, I see. The description is some what cryptic, but I think I got it,
after reading through the code.
BUT this still means that we need to do rcu_barrier() if the
SLAB_DESTROY_BY_RCU is NOT set and we do call_rcu() our self.
Correct, in that case its necessary.
My understanding for the code is (please feel free to correct me): that
if SLAB_DESTROY_BY_RCU _is_ set, then the __cache_shrink() call will
call drain_freelist(), which calls slab_destroy().
If SLAB_DESTROY_BY_RCU _is_ set, then slab_destroy() will then start a
call_rcu() callback to kmem_rcu_free() which calls kmem_cache_free().
Given that the callback code kmem_rcu_free() is not removed, we are not
worried about unloading the module at this point.
Yep, thats my understanding as well.
I'm a bit worried about what happens if __kmem_cache_destroy() is
invoked and there is still callbacks for kmem_rcu_free() in flight?
The synchronize_rcu() between __cache_shrink() and
__kmem_cache_destroy() should perhaps be changed to rcu_barrier()?
But I'm sure that the SLAB/MM guys will tell me that this case is
handled (and something about its unlinked from the appropiate
lists)??? ;-)
I'll leave that question to the MM guys :)
RCU barriers, rcu_barrier(), is inserted two places.
In nf_conntrack_expect.c nf_conntrack_expect_fini() before the
kmem_cache_destroy(), even though the use of the SLAB_DESTROY_BY_RCU
flag, because slub does not (currently) handle rcu sync correctly.
I think that should be fixed in slub then.
I don't think so, we/I'm are talking about "nf_conntrack_expect" and not
"nf_conntrack" slab. Clearly the slab "nf_conntrack" is handled
correcly (according to description above).
We still need to make sure the callbacks for "nf_conntrack_expect", are
done before unloading/removing the code they are about to call.
Yes, my response was referring to potential sl*b bugs, but
you're correct, we do need rcu_barrier() for expectations.
And in nf_conntrack_extend.c nf_ct_extend_unregister(), inorder to
wait for completion of callbacks to __nf_ct_ext_free_rcu(), which is
invoked by __nf_ct_ext_add(). It might be more efficient to call
rcu_barrier() in nf_conntrack_core.c nf_conntrack_cleanup_net(), but
thats make it more difficult to read the code (as the callback code
in located in nf_conntrack_extend.c).
This one looks fine.
Should I make two different patchs?
Either way is fine.
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