Re: [v3 PATCH net] net: enetc: fix the deadlock of enetc_mdio_lock

[Jianpeng Chang <jianpeng.chang.cn@windriver.com>]

在 2025/10/10 19:08, Vladimir Oltean 写道:
> CAUTION: This email comes from a non Wind River email account!
> Do not click links or open attachments unless you recognize the sender and know the content is safe.
>
> On Fri, Oct 10, 2025 at 01:51:38PM +0300, Vladimir Oltean wrote:
>> On Fri, Oct 10, 2025 at 12:31:37PM +0300, Wei Fang wrote:
>>>> After applying the workaround for err050089, the LS1028A platform
>>>> experiences RCU stalls on RT kernel. This issue is caused by the
>>>> recursive acquisition of the read lock enetc_mdio_lock. Here list some
>>>> of the call stacks identified under the enetc_poll path that may lead to
>>>> a deadlock:
>>>>
>>>> enetc_poll
>>>>    -> enetc_lock_mdio
>>>>    -> enetc_clean_rx_ring OR napi_complete_done
>>>>       -> napi_gro_receive
>>>>          -> enetc_start_xmit
>>>>             -> enetc_lock_mdio
>>>>             -> enetc_map_tx_buffs
>>>>             -> enetc_unlock_mdio
>>>>    -> enetc_unlock_mdio
>>>>
>>>> After enetc_poll acquires the read lock, a higher-priority writer attempts
>>>> to acquire the lock, causing preemption. The writer detects that a
>>>> read lock is already held and is scheduled out. However, readers under
>>>> enetc_poll cannot acquire the read lock again because a writer is already
>>>> waiting, leading to a thread hang.
>>>>
>>>> Currently, the deadlock is avoided by adjusting enetc_lock_mdio to prevent
>>>> recursive lock acquisition.
>>>>
>>>> Fixes: 6d36ecdbc441 ("net: enetc: take the MDIO lock only once per NAPI poll
>>>> cycle")
>>>> Signed-off-by: Jianpeng Chang <jianpeng.chang.cn@windriver.com>
>>> Acked-by: Wei Fang <wei.fang@nxp.com>
>>>
>>> Hi Vladimir,
>>>
>>> Do you have any comments? This patch will cause the regression of performance
>>> degradation, but the RCU stalls are more severe.
>>>
>> I'm fine with the change in principle. It's my fault because I didn't
>> understand how rwlock writer starvation prevention is implemented, I
>> thought there would be no problem with reentrant readers.
>>
>> But I wonder if xdp_do_flush() shouldn't also be outside the enetc_lock_mdio()
>> section. Flushing XDP buffs with XDP_REDIRECT action might lead to
>> enetc_xdp_xmit() being called, which also takes the lock...
> And I think the same concern exists for the xdp_do_redirect() calls.
> Most of the time it will be fine, but when the batch fills up it will be
> auto-flushed by bq_enqueue():
>
>          if (unlikely(bq->count == DEV_MAP_BULK_SIZE))
>                  bq_xmit_all(bq, 0);

Hi Vladimir, Wei,

If xdp_do_flush and xdp_do_redirect can potentially call enetc_xdp_xmit, 
we should move them outside of enetc_lock_mdio.

If there are no further comments, I will repost the patch with fixes for 
xdp_do_flush and xdp_do_redirect.


Thanks,

Jianpeng