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

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Message-ID: <20180119011101.GA15920@electric-eye.fr.zoreil.com>
Date:   Fri, 19 Jan 2018 02:11:01 +0100
From:   Francois Romieu <romieu@...zoreil.com>
To:     Jia-Ju Bai <baijiaju1990@...il.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

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.

-- 
Ueimor