netdev - Re: [PATCH net] net: fix race between napi kthread mode and busy poll

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Date:   Wed, 24 Feb 2021 11:48:51 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Wei Wang <weiwan@...gle.com>, Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Paolo Abeni <pabeni@...hat.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Alexander Duyck <alexanderduyck@...com>,
        Martin Zaharinov <micron10@...il.com>
Subject: Re: [PATCH net] net: fix race between napi kthread mode and busy
 poll

On Tue, 23 Feb 2021 15:41:30 -0800 Wei Wang wrote:
> Currently, napi_thread_wait() checks for NAPI_STATE_SCHED bit to
> determine if the kthread owns this napi and could call napi->poll() on
> it. However, if socket busy poll is enabled, it is possible that the
> busy poll thread grabs this SCHED bit (after the previous napi->poll()
> invokes napi_complete_done() and clears SCHED bit) and tries to poll
> on the same napi.
> This patch tries to fix this race by adding a new bit
> NAPI_STATE_SCHED_BUSY_POLL in napi->state. This bit gets set in
> napi_busy_loop() togther with NAPI_STATE_SCHED, and gets cleared in
> napi_complete_done() together with NAPI_STATE_SCHED. This helps
> distinguish the ownership of the napi between kthread and the busy poll
> thread, and prevents the kthread from polling on the napi when this napi
> is still owned by the busy poll thread.
> 
> Fixes: 29863d41bb6e ("net: implement threaded-able napi poll loop support")
> Reported-by: Martin Zaharinov <micron10@...il.com>
> Suggested-by: Alexander Duyck <alexanderduyck@...com>
> Reviewed-by: Alexander Duyck <alexanderduyck@...com>
> Reviewed-by: Eric Dumazet <edumazet@...gle.come>

AFAIU sched bit controls the ownership of the poll_list. Can we please
add a poll_list for the thread and make sure the thread polls based on
the list?
IMO that's far clearer than defining a forest of ownership state bits.

I think with just the right (wrong?) timing this patch will still not
protect against disabling the NAPI.