linux-kernel - Re: net: r8169: a question of memory barrier in the r8169 driver

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Date:   Fri, 19 Jan 2018 09:24:55 +0800
From:   Jia-Ju Bai <baijiaju1990@...il.com>
To:     Francois Romieu <romieu@...zoreil.com>
Cc:     nic_swsd@...ltek.com, alexander.h.duyck@...hat.com,
        David Miller <davem@...emloft.net>, dhowells@...hat.com,
        paulmck@...ux.vnet.ibm.com, will.deacon@....com,
        peterz@...radead.org, netdev@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: net: r8169: a question of memory barrier in the r8169 driver


On 2018/1/19 9:11, Francois Romieu wrote:
> Jia-Ju Bai <baijiaju1990@...il.com> :
> [...]
>> The function rtl8169_start_xmit reads tp->dirty_tx in TX_FRAGS_READY_FOR:
>>      if (unlikely(!TX_FRAGS_READY_FOR(tp, skb_shinfo(skb)->nr_frags))) {
>>          netif_err(tp, drv, dev, "BUG! Tx Ring full when queue awake!\n");
>>          goto err_stop_0;
>>      }
>> But there is no memory barrier around this code.
>>
>> Is there a possible data race here?
> This code would not even be needed if rtl8169_start_xmit was only your
> usual ndo_start_xmit handler: Realtek {ab / re}used it for GSO handling
> (see r8169_csum_workaround).
>
> If the test is not a no-op in this GSO context, it's racy.
>

Thanks for reply.
I didn't clearly understand your meaning...
I wonder whether there is a possible data race and whether a "smp_mb" is 
needed before this code?
By the way, do you mean that this code can be removed?


Thanks,
Jia-Ju Bai