netdev - stmmac: Race in coalesce timer and NAPI

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Date:   Fri, 21 Sep 2018 10:19:45 +0100
From:   Jose Abreu <Jose.Abreu@...opsys.com>
To:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Joao Pinto <Joao.Pinto@...opsys.com>
Subject: stmmac: Race in coalesce timer and NAPI

Hello,

I'm getting a race in stmmac coalesce timer and the
napi_schedule() interrupt and I'm asking for advice. Currently,
we are scheduling NAPI in coalesce timer but this leads to
stmmac_tx_clean() deadlock because this function tries to acquire
queue lock.

I find that this is not expected because only one instance of
NAPI should run at same time so I was wondering if it is possible
that xmit() callback is causing the deadlock ?

BTW, this is solved by:
    - Directly call stmmac_tx_clean() in timer function AND
    - Use netif_tx_trylock() in stmmac_tx_clean(). Then, if queue
is already locked we re-arm coalesce timer or reschedule NAPI.

This is easily reproducible in an ARM board with 8 core running
at 100MHz each.

Thanks and Best Regards,
Jose Miguel Abreu