Re: [PATCH net] netfilter: flowtable: Add pending bit for offload work

[Paul Blakey <paulb@mellanox.com>]

On 5/11/2020 1:14 AM, Pablo Neira Ayuso wrote:
> Hi,
>
> On Wed, May 06, 2020 at 02:24:39PM +0300, Paul Blakey wrote:
>> Gc step can queue offloaded flow del work or stats work.
>> Those work items can race each other and a flow could be freed
>> before the stats work is executed and querying it.
>> To avoid that, add a pending bit that if a work exists for a flow
>> don't queue another work for it.
>> This will also avoid adding multiple stats works in case stats work
>> didn't complete but gc step started again.
> This is happening since the mutex has been removed, right?
>
> Another question below.
it's from the using a new workqueue and one work per item, allowing parallelization
between a flow work items.


>
>> Signed-off-by: Paul Blakey <paulb@mellanox.com>
>> Reviewed-by: Roi Dayan <roid@mellanox.com>
>> ---
>>  include/net/netfilter/nf_flow_table.h | 1 +
>>  net/netfilter/nf_flow_table_offload.c | 8 +++++++-
>>  2 files changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/net/netfilter/nf_flow_table.h b/include/net/netfilter/nf_flow_table.h
>> index 6bf6965..c54a7f7 100644
>> --- a/include/net/netfilter/nf_flow_table.h
>> +++ b/include/net/netfilter/nf_flow_table.h
>> @@ -127,6 +127,7 @@ enum nf_flow_flags {
>>  	NF_FLOW_HW_DYING,
>>  	NF_FLOW_HW_DEAD,
>>  	NF_FLOW_HW_REFRESH,
>> +	NF_FLOW_HW_PENDING,
>>  };
>>  
>>  enum flow_offload_type {
>> diff --git a/net/netfilter/nf_flow_table_offload.c b/net/netfilter/nf_flow_table_offload.c
>> index b9d5ecc..731d738 100644
>> --- a/net/netfilter/nf_flow_table_offload.c
>> +++ b/net/netfilter/nf_flow_table_offload.c
>> @@ -817,6 +817,7 @@ static void flow_offload_work_handler(struct work_struct *work)
>>  			WARN_ON_ONCE(1);
>>  	}
>>  
>> +	clear_bit(NF_FLOW_HW_PENDING, &offload->flow->flags);
>>  	kfree(offload);
>>  }
>>  
>> @@ -831,9 +832,14 @@ static void flow_offload_queue_work(struct flow_offload_work *offload)
>>  {
>>  	struct flow_offload_work *offload;
>>  
>> +	if (test_and_set_bit(NF_FLOW_HW_PENDING, &flow->flags))
>> +		return NULL;
> In case of stats, it's fine to lose work.
>
> But how does this work for the deletion case? Does this falls back to
> the timeout deletion?

We get to nf_flow_table_offload_del (delete) in these cases:

>-------if (nf_flow_has_expired(flow) || nf_ct_is_dying(flow->ct) ||
>-------    test_bit(NF_FLOW_TEARDOWN, &flow->flags) {
>------->-------   ....
>------->-------    nf_flow_offload_del(flow_table, flow);

Which are all persistent once set but the nf_flow_has_expired(flow). So we will
try the delete
again and again till pending flag is unset or the flow is 'saved' by the already
queued stats updating the timeout.
A pending stats update can't save the flow once it's marked for teardown or
(flow->ct is dying), only delay it.

We didn't mention flush, like in table free. I guess we need to flush the
hardware workqueue
of any pending stats work, then queue the deletion, and flush again:

Adding nf_flow_table_offload_flush(flow_table), after
cancel_delayed_work_sync(&flow_table->gc_work);



>
> Thanks.