netdev - problem with rtnetlink 'reference' count

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Message-ID: <20171023142555.GF3165@worktop.lehotels.local>
Date:   Mon, 23 Oct 2017 16:25:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Miller <davem@...emloft.net>
Cc:     fw@...len.de, netdev@...r.kernel.org
Subject: problem with rtnetlink 'reference' count

Hi,

I just ran across commit:

  019a316992ee ("rtnetlink: add reference counting to prevent module unload while dump is in progress")

And that commit is _completely_ broken.

 1) it not in fact a refcount, so using refcount_t is silly
 2) there is a distinct lack of memory barriers, so we can easily
    observe the decrement while the msg_handler is still in progress.
 3) waiting with a schedule()/yield() loop is complete crap and subject
    life-locks, imagine doing that rtnl_unregister_all() from a RT task.

Please fix..