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

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Date:   Tue, 21 Jan 2020 15:52:58 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Finn Thain <fthain@...egraphics.com.au>
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 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
> 

Oh well... 

I would rather keep the m68k specific wrapper...

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.