Re: [PATCH] Fix kernel lockup in RTL-8169 gigabit ethernet driver

[Manfred Spraul]

Andrew wrote:

> The logic is faulty, or at least very odd.
>
> 	tx_left = tp->cur_tx - dirty_tx;
>
> 	while (tx_left > 0) {
> 		int entry = dirty_tx % NUM_TX_DESC;
>
> 		if (!(le32_to_cpu(tp->TxDescArray[entry].status) & OWNbit)) {
> 			...
> 		}
> 	}
>
> Why is that `if' test there at all?  If it ever returns false, the box
> locks up.  A BUG_ON(le32_to_cpu(tp->TxDescArray[entry].status) & OWNbit)
> might make more sense.
>  
tx_left counts packets submitted by hard_xmit_start to the hardware. 
Initially OWNbit is set, the packet is owned by the nic. The OWNbit is 
cleared by the hardware after the packet was sent. A packet with OWNbit 
set means that the nic didn't send it yet to the wire. I think the "else 
break;" patch is correct, but someone with docs should confirm that.

Adam: did you see deadlocks that disappeared after applying your patch? 
It shouldn't deadlock - it should loop until the nic sends the packet to 
the wire. It might take a few msecs, but then it should continue. 
Perhaps gcc optimized away the reload from memory and loops on a 
register. Or there is another bug that is hidden by your patch.

--
   Manfred

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