Let me add a specific point on this subject: "the API or CLI invocation stops all workers somewhere between graph nodes."
API / CLI barrier syncs stop worker threads at a very specific point: the top of the dispatch loop. At that point, all extant vectors (packets) have been fully processed. As a result, packets will not enter the forwarding graph with unintended [stale] sw_if_index values. HTH... Dave -----Original Message----- From: vpp-dev@lists.fd.io <vpp-dev@lists.fd.io> On Behalf Of Andreas Schultz Sent: Monday, June 3, 2019 11:39 AM To: vpp-dev@lists.fd.io Subject: [vpp-dev] sw_if_index reuse in tunnel implementations Hi, I'm wondering whether the current practice of reusing sw_if_indexes in some of tunnel implementations is safe. What currently happens is that when a tunnel session is released, the sw_if_index of the corresponding interface is put into vector of reuse. My first question is, why a vector? Wouldn't putting it back directly into the pool lead to the same result? Further, the vector works as LIFO, meaning the index can be reused immediately. This, at least in theory, seems to create a race condition. The API or CLI invocation stops all workers somewhere between graph nodes. That means it could stop somewhere in the pipeline with a packet that belongs to the sw interface that is about the be removed. The API call will remove the interface, but will not clear the packet from the processing pipeline. Once the workers continue, they will hit the packet with the now invalid sw interface and might try to dereference that into an invalid data structure (the pool entry is still valid, but it will be shutdown at this point and possibly contain other invalid data). The interface reuse vector will guarantee that the interface pool entry is still valid, but with a very fast API invocation or some kind of batching it could happen that the interface entry is already reused for a different session. This would lead to undefined results in the best case or a crash in the worst case. Is my above analysis correct or do I miss something? Many thanks Andreas -- -- Dipl.-Inform. Andreas Schultz ----------------------- enabling your networks ---------------------- Travelping GmbH Phone: +49-391-81 90 99 0 Roentgenstr. 13 Fax: +49-391-81 90 99 299 39108 Magdeburg Email: i...@travelping.com GERMANY Web: http://www.travelping.com Company Registration: Amtsgericht Stendal Reg No.: HRB 10578 Geschaeftsfuehrer: Holger Winkelmann VAT ID No.: DE236673780 ---------------------------------------------------------------------
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