Hi Sven,
On Wed, Feb 02, 2011 at 10:42:46PM +0100, Sven Eckelmann wrote:
gw_deselet():
- is the refcount at this time always 1 for gw_node, can the null
pointer check + a rcu_dereference be ommited? (at least that's what it looks like when comparing to the rcuref.txt example)
Why can't it be NULL? And _always_ use rcu_dereference. What example tells you that it isn't needed? None of the examples has any kind of rcu pointer in it (just el as pointer which is stored in a struct were the pointer inside the struct is rcu protected).
Ok, you got a point there with the always-rcu-dereference pointers. I somehow was thinking that in between the spin-lock/unlock there could possibly be no other thread reading/writing to it then - but I guess at that moment I forgot about the reordering and the whole point of using the rcu macros between the spinlock there :). So, yes, you're right with that one, will change it.
For the NULL pointer, guess you're right again. I was looking at the delete() example in rcuref.txt which was not doing any NULL pointer check. But either that's the case there because it's more pseudo-code there or because it's more related to lists, meaning that after the delete_element there it's not in the list anymore and not possible for any other thread to have the idea to free the same thing again.
gw_get_selected():
- Probably the orig_node's refcounting has to be made atomic, too?
This part is still a little bit ugly and I cannot give you an easy answer. Just think about following:
- Hash list is a bunch of rcu protected lists
- pointer to originator is stored inside a bucket (list elements inside the hash)
- hash bucket wants to get removed - call_rcu; reference count of the originator is decremented immediately
- (!!!! lots of reordering of read and write commands inside the cpu!!!! - aren't we happy about the added complexity which tries to hide the memory latency?)
- the originator was removed, the bucket which is removed in the call_rcu still points to the removed originator
- a parallel running operation tries to find a originator, the rcu list iterator gets the to-be-deleted bucket to the originator
- the pointer to the already removed originator inside the bucket is dereferenced, data is read/written -> Kernel Oops
Does this sound scary? At least it could be used in some horror movies (and I would watch them).
But that is the other problem I currently have with the state of batman-adv in trunk - and I think I forget to tell you about it after the release of v2011.0.0.
So, a good idea would be the removal of the buckets for the hash. Usage of "struct hlist_node" inside the hash elements should be a good starting point. But think about the problem that the different hashes could have the same element. So you need for each distinct hash an extra "struct hlist_node" inside the element which should be part of the hash. The hash_add (and related) functions don't get the actual pointer to the element, but the pointer to the correct "struct hlist_node" inside the element/struct. The comparison and hashing function would also receive "struct hlist_node" as parameter and must get the pointer to the element using the container_of macro.
@@ -171,7 +172,7 @@ struct bat_priv { struct delayed_work hna_work; struct delayed_work orig_work; struct delayed_work vis_work;
struct gw_node *curr_gw;
struct gw_node *curr_gw; /* rcu protected pointer */ struct vis_info *my_vis_info;};
Sry, but I have to say that: FAIL ;)
I think it should look that way:
struct gw_node *curr_gw;
struct gw_node __rcu *curr_gw;
Eh, had been looking at whatisRCU.txt and there gbl_foo in section 3 did not have a "__rcu" (actually I hadn't seen that in any of the documentations before).
Best regards, Sven
Cheers, Linus