Re: [PATCH net-next] net: sched: change tcf_del_walker() to use concurrent-safe delete

[Cong Wang <xiyou.wangcong@gmail.com>]

On Mon, Sep 3, 2018 at 1:33 PM Vlad Buslov <vladbu@mellanox.com> wrote:
>
>
> On Mon 03 Sep 2018 at 18:50, Cong Wang <xiyou.wangcong@gmail.com> wrote:
> > On Mon, Sep 3, 2018 at 12:06 AM Vlad Buslov <vladbu@mellanox.com> wrote:
> >>
> >> Action API was changed to work with actions and action_idr in concurrency
> >> safe manner, however tcf_del_walker() still uses actions without taking
> >> reference to them first and deletes them directly, disregarding possible
> >> concurrent delete.
> >>
> >> Change tcf_del_walker() to use tcf_idr_delete_index() that doesn't require
> >> caller to hold reference to action and accepts action id as argument,
> >> instead of direct action pointer.
> >
> > Hmm, why doesn't tcf_del_walker() just take idrinfo->lock? At least
> > tcf_dump_walker() already does.
>
> Because tcf_del_walker() calls __tcf_idr_release(), which take
> idrinfo->lock itself (deadlock). It also calls sleeping functions like

Deadlock can be easily resolved by moving the lock out.


> tcf_action_goto_chain_fini(), so just implementing function that
> releases action without taking idrinfo->lock is not enough.

Sleeping can be resolved either by making it atomic or
deferring it to a work queue.

None of your arguments here is a blocker to locking
idrinfo->lock. You really should focus on if it is really
necessary to lock idrinfo->lock in tcf_del_walker(), rather
than these details.

For me, if you need idrinfo->lock for dump walker, you must
need it for delete walker too, because deletion is a writer
which should require stronger protection than the dumper,
which merely a reader.