netdev - Re: [PATCH net] net: renesas: rswitch: Fix return value in error path of xmit

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Date: Thu, 01 Jun 2023 18:26:03 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Simon Horman <simon.horman@...igine.com>, Yoshihiro Shimoda
 <yoshihiro.shimoda.uh@...esas.com>, s.shtylyov@....ru, davem@...emloft.net,
  edumazet@...gle.com, netdev@...r.kernel.org,
 linux-renesas-soc@...r.kernel.org
Subject: Re: [PATCH net] net: renesas: rswitch: Fix return value in error
 path of xmit

On Thu, 2023-06-01 at 09:10 -0700, Jakub Kicinski wrote:
> On Thu, 01 Jun 2023 10:41:34 +0200 Paolo Abeni wrote:
> > > I agree that this is the correct return value for this case.
> > > But I do wonder if, as per the documentation of ndo_start_xmit,
> > > something should be done to avoid getting into such a situation.
> > > 
> > >  * netdev_tx_t (*ndo_start_xmit)(struct sk_buff *skb,
> > >  *                               struct net_device *dev);
> > >  *      Called when a packet needs to be transmitted.
> > >  *      Returns NETDEV_TX_OK.  Can return NETDEV_TX_BUSY, but you should stop
> > >  *      the queue before that can happen; it's for obsolete devices and weird
> > >  *      corner cases, but the stack really does a non-trivial amount
> > >  *      of useless work if you return NETDEV_TX_BUSY.
> > >  *      Required; cannot be NULL.  
> > 
> > I agree with Simon, it looks like the driver usage of
> > netif_stop_subqueue()/netif_wake_subqueue() is a dubious.
> > 
> > I think you will be better of using
> > netif_subqueue_maybe_stop()/netif_subqueue_completed_wake() alike what
> > rtl8169 is doing. e.g. netif_subqueue_maybe_stop() should be invoked
> > after the tx buffer enqueue, and netif_subqueue_completed_wake() should
> > be invoked after successful tx ring cleanup.
> 
> That's a separate issue, tho, right? The cleanup is lockless and our
> magic lockless macro scheme does not protect from spurious wakeups.
> So they still need to check if the queue is full at the top of xmit.
> And they still need to return the correct error in that case..

I guess you are right, dubious wakeup could be addresses with a
separate patch if needed. 

Fine by me to apply it as-is.

Cheers,

Paolo