linux-kernel - Re: [PATCH v2 net] net: fix premature exit from NAPI state polling in napi

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Message-ID: <CANn89i+67J4GNbWmpMf8ieFFRK5BuPRA=1Q83KU+9-gRvg+aGQ@mail.gmail.com>
Date:   Wed, 10 Nov 2021 12:11:24 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Alexander Lobakin <alexandr.lobakin@...el.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Michal Swiatkowski <michal.swiatkowski@...el.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Antoine Tenart <atenart@...nel.org>,
        Wei Wang <weiwan@...gle.com>,
        Björn Töpel <bjorn@...nel.org>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 net] net: fix premature exit from NAPI state polling in napi_disable()

On Wed, Nov 10, 2021 at 11:56 AM Alexander Lobakin
<alexandr.lobakin@...el.com> wrote:
>
> Commit 719c57197010 ("net: make napi_disable() symmetric with
> enable") accidentally introduced a bug sometimes leading to a kernel
> BUG when bringing an iface up/down under heavy traffic load.
>
> Prior to this commit, napi_disable() was polling n->state until
> none of (NAPIF_STATE_SCHED | NAPIF_STATE_NPSVC) is set and then
> always flip them. Now there's a possibility to get away with the
> NAPIF_STATE_SCHE unset as 'continue' drops us to the cmpxchg()
> call with an unitialized variable, rather than straight to
> another round of the state check.
>

...

>
> [0] https://lore.kernel.org/netdev/20211110191126.1214-1-alexandr.lobakin@intel.com
>
> Fixes: 719c57197010 ("net: make napi_disable() symmetric with enable")
> Suggested-by: Eric Dumazet <edumazet@...gle.com> # for-loop
> Signed-off-by: Alexander Lobakin <alexandr.lobakin@...el.com>
> Reviewed-by: Jesse Brandeburg <jesse.brandeburg@...el.com>
> ---

Thanks a lot !
Reviewed-by: Eric Dumazet <edumazet@...gle.com>