Paul E. McKenney wrote:
quoted text > O
..snip
quoted text >> Hum... Another way of handling all those cases and avoid memory barriers
>> would be to have different "NULL" pointers.
>>
>> Each hash chain should have a unique "NULL" pointer (in the case of UDP, it
>> can be the 128 values : [ (void*)0 .. (void *)127 ]
>>
>> Then, when performing a lookup, a reader should check the "NULL" pointer
>> it get at the end of its lookup has is the "hash" value of its chain.
>>
>> If not -> restart the loop, aka "goto begin;" :)
>>
>> We could avoid memory barriers then.
>>
>> In the two cases Corey mentioned, this trick could let us avoid memory
>> barriers.
>> (existing one in sk_add_node_rcu(sk, &hslot->head); should be enough)
>>
>> What do you think ?
>>
>
> Kinky!!! ;-)
>
My thought exactly ;-).
quoted text > Then the rcu_dereference() would be supplying the needed memory barriers.
>
> Hmmm... I guess that the only confusion would be if the element got
> removed and then added to the same list. But then if its pointer was
> pseudo-NULL, then that would mean that all subsequent elements had been
> removed, and all preceding ones added after the scan started.
>
> Which might well be harmless, but I must defer to you on this one at
> the moment.
>
I believe that is harmless, as re-scanning the same data should be fine.
quoted text > If you need a larger hash table, another approach would be to set the
> pointer's low-order bit, allowing the upper bits to be a full-sized
> index -- or even a pointer to the list header. Just make very sure
> to clear the pointer when freeing, or an element on the freelist
> could end up looking like a legitimate end of list... Which again
> might well be safe, but why inflict this on oneself?
>
Kind of my thought, too. That's a lot of work to avoid a single
smb_wmb() on the socket creation path. Plus this could be extra confusing.
-corey
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