Re: [Patch net v3 3/3] net_sched: carefully handle tcf_block_put()

[Jiri Pirko <jiri@resnulli.us>]

Tue, Sep 12, 2017 at 01:33:32AM CEST, xiyou.wangcong@gmail.com wrote:
>As pointed out by Jiri, there is still a race condition between
>tcf_block_put() and tcf_chain_destroy() in a RCU callback. There
>is no way to make it correct without proper locking or synchronization,
>because both operate on a shared list.
>
>Locking is hard, because the only lock we can pick here is a spinlock,
>however, in tc_dump_tfilter() we iterate this list with a sleeping
>function called (tcf_chain_dump()), which makes using a lock to protect
>chain_list almost impossible.
>
>Jiri suggested the idea of holding a refcnt before flushing, this works
>because it guarantees us there would be no parallel tcf_chain_destroy()
>during the loop, therefore the race condition is gone. But we have to
>be very careful with proper synchronization with RCU callbacks.
>
>Suggested-by: Jiri Pirko <jiri@mellanox.com>
>Cc: Jamal Hadi Salim <jhs@mojatatu.com>
>Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>

Acked-by: Jiri Pirko <jiri@mellanox.com>

Thanks!