netdev - Re: [PATCH wpan-next v2 05/11] net: mac802154: Bring the hability to hold the transmit queue

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Date:   Tue, 17 May 2022 11:27:26 +0200
From:   Miquel Raynal <miquel.raynal@...tlin.com>
To:     Alexander Aring <alex.aring@...il.com>
Cc:     Stefan Schmidt <stefan@...enfreihafen.org>,
        linux-wpan - ML <linux-wpan@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "open list:NETWORKING [GENERAL]" <netdev@...r.kernel.org>,
        David Girault <david.girault@...vo.com>,
        Romuald Despres <romuald.despres@...vo.com>,
        Frederic Blain <frederic.blain@...vo.com>,
        Nicolas Schodet <nico@...fr.eu.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH wpan-next v2 05/11] net: mac802154: Bring the hability
 to hold the transmit queue

Hi Alex,

> > @@ -84,7 +118,7 @@ void ieee802154_xmit_complete(struct ieee802154_hw *hw, struct sk_buff *skb,
> >                                       hw->phy->sifs_period * NSEC_PER_USEC,
> >                                       HRTIMER_MODE_REL);
> >         } else {
> > -               ieee802154_wake_queue(hw);
> > +               ieee802154_release_queue(local);
> >         }
> >
> >         dev_consume_skb_any(skb);
> > @@ -98,7 +132,7 @@ void ieee802154_xmit_error(struct ieee802154_hw *hw, struct sk_buff *skb,
> >         struct ieee802154_local *local = hw_to_local(hw);
> >
> >         local->tx_result = reason;
> > -       ieee802154_wake_queue(hw);
> > +       ieee802154_release_queue(local);
> >         dev_kfree_skb_any(skb);
> >         atomic_dec(&hw->phy->ongoing_txs);  
> 
> I am pretty sure that will end in a scheduling while atomic warning
> with hwsim. If you don't hit it you have the wrong config, you need to
> enable such warnings and have the right preemption model setting.

I was using the "desktop" kernel preemption model (voluntary), I've
switched to CONFIG_PREEMPT ("Preemptible kernel (Low-latency)"),
and enabled CONFIG_DEBUG_ATOMIC_SLEEP. You are right that we should use
a spinlock instead of a mutex here. However I don't think disabling
IRQs is necessary, so I'll switch to spin_(un)lock() calls.

> These calls xmit complete/error should even be allowed to be called
> from a hardware irq context, however I _think_ we don't have a driver
> which currently does that, but the mutex will break stuff here in the
> xmit_do() callback of netdev which hwsim is calling it from.
> 
> Please check again...

Will perform a new set of test runs with the new configuration, yes.

Thanks,
Miquèl