netdev - Re: [Patch net v3 3/3] net_sched: carefully handle tcf_block

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Message-ID: <20170912104358.GG2036@nanopsycho>
Date:   Tue, 12 Sep 2017 12:43:58 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     netdev@...r.kernel.org, jiri@...lanox.com,
        jakub.kicinski@...ronome.com, jhs@...atatu.com
Subject: Re: [Patch net v3 3/3] net_sched: carefully handle tcf_block_put()

Tue, Sep 12, 2017 at 01:33:32AM CEST, xiyou.wangcong@...il.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@...lanox.com>
>Cc: Jamal Hadi Salim <jhs@...atatu.com>
>Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>

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

Thanks!