netdev - Re: [PATCH net] ixgbe: check return value of napi_complete

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Message-ID: <c1251d5d-4b18-a1de-acd2-6de829b7c7ac@gmail.com>
Date:   Thu, 20 Sep 2018 13:35:00 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Song Liu <songliubraving@...com>, netdev@...r.kernel.org
Cc:     intel-wired-lan@...ts.osuosl.org, kernel-team@...com,
        stable@...r.kernel.org, Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Subject: Re: [PATCH net] ixgbe: check return value of napi_complete_done()



On 09/20/2018 12:01 PM, Song Liu wrote:
> The NIC driver should only enable interrupts when napi_complete_done()
> returns true. This patch adds the check for ixgbe.
> 
> Cc: stable@...r.kernel.org # 4.10+
> Cc: Jeff Kirsher <jeffrey.t.kirsher@...el.com>
> Suggested-by: Eric Dumazet <edumazet@...gle.com>
> Signed-off-by: Song Liu <songliubraving@...com>
> ---


Well, unfortunately we do not know why this is needed,
this is why I have not yet sent this patch formally.

netpoll has correct synchronization :

poll_napi() places into napi->poll_owner current cpu number before calling poll_one_napi()

netpoll_poll_lock() does also use napi->poll_owner

When netpoll calls ixgbe poll() method, it passed a budget of 0,
meaning napi_complete_done() is not called.

As long as we can not explain the problem properly in the changelog,
we should investigate, otherwise we will probably see coming dozens of patches
trying to fix a 'potential hazard'.

Thanks.