netdev - Re: [PATCH net v2 01/12] net/sonic: Add mutual exclusion for accessing shared state

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Message-ID: <alpine.LNX.2.21.1.2001221139480.8@nippy.intranet>
Date:   Wed, 22 Jan 2020 11:40:09 +1100 (AEDT)
From:   Finn Thain <fthain@...egraphics.com.au>
To:     Eric Dumazet <eric.dumazet@...il.com>
cc:     "David S. Miller" <davem@...emloft.net>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Chris Zankel <chris@...kel.net>,
        Laurent Vivier <laurent@...ier.eu>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v2 01/12] net/sonic: Add mutual exclusion for accessing
 shared state

On Tue, 21 Jan 2020, Eric Dumazet wrote:

> On 1/21/20 3:33 PM, Finn Thain wrote:
> > On Tue, 21 Jan 2020, Eric Dumazet wrote:
> > 
> >> On 1/21/20 1:22 PM, Finn Thain wrote:
> >>> The netif_stop_queue() call in sonic_send_packet() races with the
> >>> netif_wake_queue() call in sonic_interrupt(). This causes issues
> >>> like "NETDEV WATCHDOG: eth0 (macsonic): transmit queue 0 timed out".
> >>> Fix this by disabling interrupts when accessing tx_skb[] and next_tx.
> >>> Update a comment to clarify the synchronization properties.
> >>>
> >>> Fixes: efcce839360f ("[PATCH] macsonic/jazzsonic network drivers update")
> >>> Tested-by: Stan Johnson <userm57@...oo.com>
> >>> Signed-off-by: Finn Thain <fthain@...egraphics.com.au>
> >>
> >>> @@ -284,9 +287,16 @@ static irqreturn_t sonic_interrupt(int irq, void *dev_id)
> >>>  	struct net_device *dev = dev_id;
> >>>  	struct sonic_local *lp = netdev_priv(dev);
> >>>  	int status;
> >>> +	unsigned long flags;
> >>> +
> >>> +	spin_lock_irqsave(&lp->lock, flags);
> >>
> >>
> >> This is a hard irq handler, no need to block hard irqs.
> >>
> >> spin_lock() here is enough.
> >>
> > 
> > Well, yes, assuming we're dealing with SMP [1]. Probably just disabling 
> > pre-emption is all that will ever be needed.
> > 
> > Anyway, the real problem solved by disabling irqs is that macsonic must 
> > avoid re-entrance of sonic_interrupt(). [2]
> > 
> > [1]
> > https://lore.kernel.org/netdev/alpine.LNX.2.21.1.2001211026190.8@nippy.intranet/T/#m0523c8b2a26a410ed56889d9230c37ba1160d40a
> > 
> > [2]
> > https://lore.kernel.org/netdev/alpine.LNX.2.21.1.2001211026190.8@nippy.intranet/T/#m1c8ca580d2b45e61a628d17839978d0bd5aaf061
> > 
> 

I was overlooking the fact that sonic_send_packet() really does have to 
exclude sonic_interrupt(). So disabling irqs is mandatory here, right?

> Oh well... 
> 
> I would rather keep the m68k specific wrapper...
> 

If disabling irqs is unavoidable, the wrapper is redundant.

> Please add a fat comment of why spin_lock_irqsave() is used, to avoid a 
> future 'cleanup', since average network developer wont be aware of m68k 
> oddities.
> 

OK.