On Thu, Oct 3, 2019 at 6:19 PM Vlad Buslov <vla...@mellanox.com> wrote: > > > On Thu 03 Oct 2019 at 19:59, John Hurley <john.hur...@netronome.com> wrote: > > 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'. > > Maybe something like:
Ok, I'll need to give this more thought. Initially it does sound like overkill. > > 1. Extend block with hash table with key being chain id and prio > combined and value is some structure that contains struct completion > (completed in tcf_proto_destroy() where we sure that all rules were > removed from hw) and a reference counter. > Maybe it could live in each chain rather than block to be more fine grained? Or would this potentially cause a similar issue on deletion of chains? > 2. When cls API wants to delete proto instance > (tcf_chain_tp_delete_empty(), chain flush, etc.), new member is added to > table from 1. with chain+prio of proto that is being deleted (atomically > with detaching of proto from chain). > > 3. When inserting new proto, verify that there are no corresponding > entry in hash table with same chain+prio. If there is, increment > reference counter and wait for completion. Release reference counter > when completed. How would the 'wait' work? Loop back via replay flag? It feels a bit like we are adding a lot more complexity to this and almost hacking something in to work around a (relatively) newly introduced problem. > > > > >> > >> Regards, > >> Vlad
