netdev - Re: BQL support in gianfar causes network hiccup

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Date:	Tue, 03 Sep 2013 08:42:20 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Per Dalén <per.dalen@...eartv.com>
Cc:	Claudiu Manoil <claudiu.manoil@...escale.com>,
	netdev@...r.kernel.org
Subject: Re: BQL support in gianfar causes network hiccup

On Tue, 2013-09-03 at 09:55 +0200, Per Dalén wrote:

> Yes, it's weird. The only reason I removed the BQL commit 
> (d8a0f1b0af67679bba886784de10d8c21acc4e0e) was because the error Tino 
> Keitel had was the similar as mine.


I suspect a genuine race in this driver. BQL only makes this race happen
more often.

gfar_poll_sq() has the following :

/* run Tx cleanup to completion */
if (tx_queue->tx_skbuff[tx_queue->skb_dirtytx])
        gfar_clean_tx_ring(tx_queue);

While gfar_poll() has a different method :

if (tx_queue->tx_skbuff[tx_queue->skb_dirtytx]) { 
    gfar_clean_tx_ring(tx_queue);
    has_tx_work = 1;
}

Note the has_tx_work use in gfar_poll() only.

Note that memory barriers seems to be missing.

1) In your cases, is it gfar_poll_sq() or gfar_poll() that is used ?

2) Is the bug happening if only one CPU is used ?



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