On Wed, Oct 29, 2008 at 01:28:15PM -0500, Corey Minyard wrote:

Eric Dumazet wrote:

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Corey Minyard a écrit :

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Paul E. McKenney wrote:

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On Wed, Oct 29, 2008 at 05:09:53PM +0100, Eric Dumazet wrote:
 

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Corey Minyard a écrit :
  

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Eric Dumazet wrote:
    

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Corey Minyard found a race added in commit 
271b72c7fa82c2c7a795bc16896149933110672d
(udp: RCU handling for Unicast packets.)

"If the socket is moved from one list to another list in-between the 
time  the hash is calculated and the next field is accessed, and the 
socket  has moved to the end of the new list, the traversal will not 
complete  properly on the list it should have, since the socket will 
be on the end  of the new list and there's not a way to tell it's on 
a new list and  restart the list traversal.  I think that this can be 
solved by  pre-fetching the "next" field (with proper barriers) 
before checking the  hash."

This patch corrects this problem, introducing a new 
sk_for_each_rcu_safenext()
macro.
        

You also need the appropriate smp_wmb() in udp_lib_get_port() after 
sk_hash is set, I think, so the next field is guaranteed to be changed 
after the hash value is changed.
      

Not sure about this one Corey.

If a reader catches previous value of item->sk_hash, two cases are to 
be taken into :

1) its udp_hashfn(net, sk->sk_hash) is != hash   -> goto begin : Reader 
will redo its scan

2) its udp_hashfn(net, sk->sk_hash) is == hash
 -> next pointer is good enough : it points to next item in same hash 
chain.
    No need to rescan the chain at this point.
    Yes we could miss the fact that a new port was bound and this UDP 
message could be lost.
    

3) its udp_hashfn(net, sk-sk_hash) is == hash, but only because it was
removed, freed, reallocated, and then readded with the same hash value,
possibly carrying the reader to a new position in the same list.
  

If I understand this, without the smp_wmb(), it is possible that the next 
field can be written to main memory before the hash value is written.  If 
that happens, the following can occur:

 CPU1                    CPU2
 next is set to NULL (end of new list)

Well, if this item is injected to the same chain, next wont be set to 
NULL.

That would mean previous writers deleted all items from the chain.

I put my comment in the wrong place, I wasn't talking about being injected 
into the same chain.

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In this case, readers can see NULL, it is not a problem at all.
List is/was empty.
An application cannot complain a packet is not
handled if its bind() syscall is not yet completed :)

If item is injected on another chain, we will detect hash mismatch and 
redo full scan.

If the item is injected onto the end of another chain, the next field will 
be NULL and you won't detect a hash mismatch.  It's basically the same 
issue as the previous race, though a lot more subtle and unlikely.  If you 
get (from the previous socket) an old value of "sk_hash" and (from the new 
socket) a new value of "next" that is NULL, you will terminate the loop 
when you should have restarted it.  I'm pretty sure that can occur without 
the write barrier.

One way of dealing with this is to keep a tail pointer.  Then, if the
element containing the NULL pointer doesn't match the tail pointer seen
at the start of the search, or if the tail pointer has changed,
restart the search.  Memory barriers will be needed.  ;-)

							Thanx, Paul

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                         fetch next
                         calculate hash and compare to sk_hash
 sk_hash is set to new value

So I think in the above cases, your case #2 is not necessarily valid 
without the barrier.

And another possible issue.  If sk_hash is written before next, and CPU1 
is interrupted before CPU2, CPU2 will continually spin on the list until 
CPU1 comes back and moves it to the new list.  Note sure if that is an 
issue.

Probably not. Previously, readers were spining on read_lock(), when a 
writer was inside its critical section (write_lock()/write_unlock()).
So instead of spining inside read_unlock(), issuing stupid memory 
transactions, the readers can now spin reading hash chain and populate
cpu cache :)

Yes, I thought about that and thought I would point it out, but I agree, 
what you have is certainly better than spinning on a lock :).


-corey