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Subject: Re: net: r8169: a question of memory barrier in the r8169 driver
To:     Francois Romieu <romieu@fr.zoreil.com>
Cc:     nic_swsd@realtek.com, alexander.h.duyck@redhat.com,
        David Miller <davem@davemloft.net>, dhowells@redhat.com,
        paulmck@linux.vnet.ibm.com, will.deacon@arm.com,
        peterz@infradead.org, netdev@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
References: <9a373156-41e5-a78b-cd31-c4b9bdba2696@gmail.com>
 <20180119011101.GA15920@electric-eye.fr.zoreil.com>
From:   Jia-Ju Bai <baijiaju1990@gmail.com>
Message-ID: <31c256b2-b527-89b1-168b-b2a529811d74@gmail.com>
Date:   Fri, 19 Jan 2018 09:24:55 +0800
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On 2018/1/19 9:11, Francois Romieu wrote:
> Jia-Ju Bai <baijiaju1990@gmail.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