Corey Minyard a écrit :

Paul E. McKenney wrote:

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O

..snip

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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 ;-).

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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.

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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.

Sure this smp_wmb() seems harmless (but, remember this infrastructure will
next be deployed for TCP sockets as well ;) )

But saving smp_rmb() at lookup time, for each item is a clear win, no ?