netdev - net: r8169: a question of memory barrier in the r8169 driver

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Message-ID: <9a373156-41e5-a78b-cd31-c4b9bdba2696@gmail.com>
Date:   Thu, 18 Jan 2018 22:06:17 +0800
From:   Jia-Ju Bai <baijiaju1990@...il.com>
To:     nic_swsd@...ltek.com, romieu@...zoreil.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
Cc:     netdev@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: net: r8169: a question of memory barrier in the r8169 driver

In the rt8169 driver, the function "rtl_tx" uses "smp_mb" to sync the 
writing operation with rtl8169_start_xmit:
     if (tp->dirty_tx != dirty_tx) {
         tp->dirty_tx = dirty_tx;
         smp_mb();
         ...
     }
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?
If not, how this data race is avoided?


Thanks,
Jia-Ju Bai