Re: [PATCH net] ixgbe: fix double clean of tx descriptors with xdp

[Ilya Maximets]

On 21.08.2019 4:17, Alexander Duyck wrote:
> On Tue, Aug 20, 2019 at 8:58 AM Ilya Maximets <i.maximets@xxxxxxxxxxx> wrote:
>>
>> On 20.08.2019 18:35, Alexander Duyck wrote:
>>> On Tue, Aug 20, 2019 at 8:18 AM Ilya Maximets <i.maximets@xxxxxxxxxxx> wrote:
>>>>
>>>> Tx code doesn't clear the descriptor status after cleaning.
>>>> So, if the budget is larger than number of used elems in a ring, some
>>>> descriptors will be accounted twice and xsk_umem_complete_tx will move
>>>> prod_tail far beyond the prod_head breaking the comletion queue ring.
>>>>
>>>> Fix that by limiting the number of descriptors to clean by the number
>>>> of used descriptors in the tx ring.
>>>>
>>>> Fixes: 8221c5eba8c1 ("ixgbe: add AF_XDP zero-copy Tx support")
>>>> Signed-off-by: Ilya Maximets <i.maximets@xxxxxxxxxxx>
>>>
>>> I'm not sure this is the best way to go. My preference would be to
>>> have something in the ring that would prevent us from racing which I
>>> don't think this really addresses. I am pretty sure this code is safe
>>> on x86 but I would be worried about weak ordered systems such as
>>> PowerPC.
>>>
>>> It might make sense to look at adding the eop_desc logic like we have
>>> in the regular path with a proper barrier before we write it and after
>>> we read it. So for example we could hold of on writing the bytecount
>>> value until the end of an iteration and call smp_wmb before we write
>>> it. Then on the cleanup we could read it and if it is non-zero we take
>>> an smp_rmb before proceeding further to process the Tx descriptor and
>>> clearing the value. Otherwise this code is going to just keep popping
>>> up with issues.
>>
>> But, unlike regular case, xdp zero-copy xmit and clean for particular
>> tx ring always happens in the same NAPI context and even on the same
>> CPU core.
>>
>> I saw the 'eop_desc' manipulations in regular case and yes, we could
>> use 'next_to_watch' field just as a flag of descriptor existence,
>> but it seems unnecessarily complicated. Am I missing something?
>>
> 
> So is it always in the same NAPI context?. I forgot, I was thinking
> that somehow the socket could possibly make use of XDP for transmit.

AF_XDP socket only triggers tx interrupt on ndo_xsk_async_xmit() which
is used in zero-copy mode. Real xmit happens inside
ixgbe_poll()
 -> ixgbe_clean_xdp_tx_irq()
    -> ixgbe_xmit_zc()

This should be not possible to bound another XDP socket to the same netdev
queue.

It also possible to xmit frames in xdp_ring while performing XDP_TX/REDIRECT
actions. REDIRECT could happen from different netdev with different NAPI
context, but this operation is bound to specific CPU core and each core has
its own xdp_ring.

However, I'm not an expert here.
BjÃrn, maybe you could comment on this?

> 
> As far as the logic to use I would be good with just using a value you
> are already setting such as the bytecount value. All that would need
> to happen is to guarantee that the value is cleared in the Tx path. So
> if you clear the bytecount in ixgbe_clean_xdp_tx_irq you could
> theoretically just use that as well to flag that a descriptor has been
> populated and is ready to be cleaned. Assuming the logic about this
> all being in the same NAPI context anyway you wouldn't need to mess
> with the barrier stuff I mentioned before.

Checking the number of used descs, i.e. next_to_use - next_to_clean,
makes iteration in this function logically equal to the iteration inside
'ixgbe_xsk_clean_tx_ring()'. Do you think we need to change the later
function too to follow same 'bytecount' approach? I don't like having
two different ways to determine number of used descriptors in the same file.

Best regards, Ilya Maximets.