netdev - Re: [PATCH net] net: core: avoid napi

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Date:   Thu, 18 Mar 2021 10:22:23 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Lijun Pan <ljp@...ux.ibm.com>
Cc:     netdev <netdev@...r.kernel.org>, Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...emloft.net>, tlfalcon@...ux.ibm.com,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>, Wei Wang <weiwan@...gle.com>,
        Cong Wang <cong.wang@...edance.com>,
        Taehee Yoo <ap420073@...il.com>,
        shemminger@...ux-foundation.org
Subject: Re: [PATCH net] net: core: avoid napi_disable to cause deadlock

On Thu, Mar 18, 2021 at 9:04 AM Lijun Pan <ljp@...ux.ibm.com> wrote:
>
> There are chances that napi_disable is called twice by NIC driver.


???

Please fix the buggy driver, or explain why it can not be fixed.


> This could generate deadlock. For example,
> the first napi_disable will spin until NAPI_STATE_SCHED is cleared
> by napi_complete_done, then set it again.
> When napi_disable is called the second time, it will loop infinitely
> because no dev->poll will be running to clear NAPI_STATE_SCHED.
>
> CPU0                            CPU1
>  napi_disable
>   test_and_set_bit
>   (napi_complete_done clears
>    NAPI_STATE_SCHED, ret 0,
>    and set NAPI_STATE_SCHED)
>                                 napi_disable
>                                  test_and_set_bit
>                                  (ret 1 and loop infinitely because
>                                   no napi instance is scheduled to
>                                   clear NAPI_STATE_SCHED bit)
>
> Checking the napi state bit to make sure if napi is already disabled,
> exit the call early enough to avoid spinning infinitely.
>
> Fixes: bea3348eef27 ("[NET]: Make NAPI polling independent of struct net_device objects.")
> Signed-off-by: Lijun Pan <ljp@...ux.ibm.com>
> ---
>  net/core/dev.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)