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, 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. Regards, Vlad
