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

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Date:   Sat, 20 Jan 2018 00:59:53 +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> :
> 
> 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...

It's fine.

> 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?

This code may be removed in a driver that properly stops itself its
tx queueing in the ndo_start_xmit handler (I would still keep it as
a bug detection helper but it's just a matter of taste). That's what
the r8169 driver used to aim at.

However, since e974604b453e87f8d864371786375d3d511fdf56, there is a piece
of code where the r8169 driver iteratively uses its own ndo_start_xmit
(without even checking its return value) in r8169_csum_workaround.
It is racy. Now, let's forget races for a few seconds: how is
r8169_csum_workaround supposed to work at all given that it does not care
if (the "unlikely(...)" test in) rtl8169_start_xmit succeeds or not ?

rtl8169_start_xmit can leave the skb as-is or map it to hardware descriptors
(whence late release in rtl_tx). net/core/dev.c::dev_hard_start_xmit cares.
r8169_csum_workaround doesn't.

-- 
Ueimor