netdev - Re: [PATCH net v2] napi: fix race inside napi

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Date:   Mon, 18 Oct 2021 17:01:27 -0700
From:   Sukadev Bhattiprolu <sukadev@...ux.ibm.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Dany Madden <drt@...ux.ibm.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, netdev@...r.kernel.org,
        linyunsheng@...wei.com, "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Antoine Tenart <atenart@...nel.org>,
        Alexander Lobakin <alobakin@...me>,
        Wei Wang <weiwan@...gle.com>, Taehee Yoo <ap420073@...il.com>,
        Björn Töpel <bjorn@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Neil Horman <nhorman@...hat.com>,
        Dust Li <dust.li@...ux.alibaba.com>
Subject: Re: [PATCH net v2] napi: fix race inside napi_enable

Jakub Kicinski [kuba@...nel.org] wrote:
> On Mon, 18 Oct 2021 16:36:36 -0700 Dany Madden wrote:
> > > The BUG_ON() is here to make sure that when napi_enable() is called the
> > > napi instance was dormant, i.e. disabled. We have "STATE_SCHED" bit set
> > > on disabled NAPIs because that bit means ownership. Whoever disabled
> > > the NAPI owns it.
> > > 
> > > That BUG_ON() could have been taken outside of the loop, there's no
> > > point re-checking on every try.
> > > 
> > > Are you seeing NAPI-related failures? We had at least 3 reports in the
> > > last two weeks of strange failures which look like NAPI state getting
> > > corrupted on net-next...  
> > 
> > We hit two napi related crashes while attempting mtu size change.

BTW these are with a couple of bug fixes in ibmvnic driver applied
to 1e0083bd0777 ("gve: DQO: avoid unused variable warnings").

We are trying to narrow it down to the following change that is
required to be able to change mtu on ibmvnic.

> 
> Is it reproducible or happens rarely and randomly?

Random. We have been testing for a couple of weeks but hit both today.

Sukadev

---
index 8f17096e614d..a1533979c670 100644
--- a/drivers/net/ethernet/ibm/ibmvnic.c
+++ b/drivers/net/ethernet/ibm/ibmvnic.c
@@ -1914,8 +1914,6 @@ static netdev_tx_t ibmvnic_xmit(struct sk_buff *skb, struct net_device *netdev)
        ind_bufp = &tx_scrq->ind_buf;
 
        if (test_bit(0, &adapter->resetting)) {
-               if (!netif_subqueue_stopped(netdev, skb))
-                       netif_stop_subqueue(netdev, queue_num);
                dev_kfree_skb_any(skb);