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

[Jianpeng Chang]

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@xxxxxxxxxxxxx>
> > > Acked-by: Wei Fang <wei.fang@xxxxxxx>
> > >
> > > 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