On Thu, Oct 3, 2019 at 5:26 PM Vlad Buslov <vla...@mellanox.com> wrote: > > > On Thu 03 Oct 2019 at 02:14, John Hurley <john.hur...@netronome.com> wrote: > > Hi, > > > > Putting this out an RFC built on net-next. It fixes some issues > > discovered in testing when using the TC API of OvS to generate flower > > rules and subsequently offloading them to HW. Rules seen contain the same > > match fields or may be rule modifications run as a delete plus an add. > > We're seeing race conditions whereby the rules present in kernel flower > > are out of sync with those offloaded. Note that there are some issues > > that will need fixed in the RFC before it becomes a patch such as > > potential races between releasing locks and re-taking them. However, I'm > > putting this out for comments or potential alternative solutions. > > > > The main cause of the races seem to be in the chain table of cls_api. If > > a tcf_proto is destroyed then it is removed from its chain. If a new > > filter is then added to the same chain with the same priority and protocol > > a new tcf_proto will be created - this may happen before the first is > > fully removed and the hw offload message sent to the driver. In cls_flower > > this means that the fl_ht_insert_unique() function can pass as its > > hashtable is associated with the tcf_proto. We are then in a position > > where the 'delete' and the 'add' are in a race to get offloaded. We also > > noticed that doing an offload add, then checking if a tcf_proto is > > concurrently deleting, then remove the offload if it is, can extend the > > out of order messages. Drivers do not expect to get duplicate rules. > > However, the kernel TC datapath they are not duplicates so we can get out > > of sync here. > > > > The RFC fixes this by adding a pre_destroy hook to cls_api that is called > > when a tcf_proto is signaled to be destroyed but before it is removed from > > its chain (which is essentially the lock for allowing duplicates in > > flower). Flower then uses this new hook to send the hw delete messages > > from tcf_proto destroys, preventing them racing with duplicate adds. It > > also moves the check for 'deleting' to before the sending the hw add > > message. > > > > John Hurley (2): > > net: sched: add tp_op for pre_destroy > > net: sched: fix tp destroy race conditions in flower > > > > include/net/sch_generic.h | 3 +++ > > net/sched/cls_api.c | 29 ++++++++++++++++++++++++- > > net/sched/cls_flower.c | 55 > > ++++++++++++++++++++++++++--------------------- > > 3 files changed, 61 insertions(+), 26 deletions(-) > > Hi John, > > Thanks for working on this! > > Are there any other sources for race conditions described in this > letter? When you describe tcf_proto deletion you say "main cause" but > don't provide any others. If tcf_proto is the only problematic part,
Hi Vlad, Thanks for the input. The tcf_proto deletion was the cause from the tests we ran. That's not to say there are not more I wasn't seeing in my analysis. > then it might be worth to look into alternative ways to force concurrent > users to wait for proto deletion/destruction to be properly finished. > Maybe having some table that maps chain id + prio to completion would be > simpler approach? With such infra tcf_proto_create() can wait for > previous proto with same prio and chain to be fully destroyed (including > offloads) before creating a new one. I think a problem with this is that the chain removal functions call tcf_proto_put() (which calls destroy when ref is 0) so, if other concurrent processes (like a dump) have references to the tcf_proto then we may not get the hw offload even by the time the chain deletion function has finished. We would need to make sure this was tracked - say after the tcf_proto_destroy function has completed. How would you suggest doing the wait? With a replay flag as happens in some other places? To me it seems the main problem is that the tcf_proto being in a chain almost acts like the lock to prevent duplicates filters getting to the driver. We need some mechanism to ensure a delete has made it to HW before we release this 'lock'. > > Regards, > Vlad
