On Fri, Oct 4, 2019 at 4:58 PM Vlad Buslov <vla...@mellanox.com> wrote: > > > On Fri 04 Oct 2019 at 18:39, John Hurley <john.hur...@netronome.com> wrote: > > 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? > > IMO just having one per block is straightforward enough, unless there is > a reason to make it per chain. > > > > >> 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? > > What is "loop back via replay flag"?
Ok, bad description :) I was referring the EAGAIN error returns used in other places post RTNL removal. e.g. https://elixir.bootlin.com/linux/v5.4-rc1/source/net/sched/cls_flower.c#L1606 > Anyway, I was suggesting to use struct completion from completion.h, > which has following functions in its API: > thanks for the pointer. > /** > * wait_for_completion: - waits for completion of a task > * @x: holds the state of this particular completion > * > * This waits to be signaled for completion of a specific task. It is NOT > * interruptible and there is no timeout. > * > * See also similar routines (i.e. wait_for_completion_timeout()) with timeout > * and interrupt capability. Also see complete(). > */ > void __sched wait_for_completion(struct completion *x) > > /** > * complete_all: - signals all threads waiting on this completion > * @x: holds the state of this particular completion > * > * This will wake up all threads waiting on this particular completion event. > * > * If this function wakes up a task, it executes a full memory barrier before > * accessing the task state. > * > * Since complete_all() sets the completion of @x permanently to done > * to allow multiple waiters to finish, a call to reinit_completion() > * must be used on @x if @x is to be used again. The code must make > * sure that all waiters have woken and finished before reinitializing > * @x. Also note that the function completion_done() can not be used > * to know if there are still waiters after complete_all() has been called. > */ > void complete_all(struct completion *x) > > > > 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. > > I'm not insisting on any particular approach, just suggesting something > which in my opinion is easier to implement than reshuffling locking in > flower and directly targets the problem you described in cover letter - > new filters can be inserted while concurrent destruction of tp with same > chain id and prio is in progress. > > > > >> > >> > > >> >> > >> >> Regards, > >> >> Vlad
