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

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Message-ID: <20180120000012.GB14475@electric-eye.fr.zoreil.com>
Date:   Sat, 20 Jan 2018 01:00:12 +0100
From:   Francois Romieu <romieu@...zoreil.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jia-Ju Bai <baijiaju1990@...il.com>, 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, 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

Peter Zijlstra <peterz@...radead.org> :
> On Fri, Jan 19, 2018 at 02:11:18AM +0100, Francois Romieu wrote:
> > Peter Zijlstra <peterz@...radead.org> :
> > [...]
> > > There is only 1 variable afaict. Memory barriers need at least 2 in
> > > order to be able to do _anything_.
> > 
> > I don't get your point: why don't {cur_tx, dirty_tx} qualify as said
> > two variables ?
> 
> There wasn't any cur_tx in the code you provided.

/* A skbuff with nr_frags needs nr_frags+1 entries in the tx queue */
#define TX_FRAGS_READY_FOR(tp,nr_frags) \
        (TX_SLOTS_AVAIL(tp) >= (nr_frags + 1))

#define TX_SLOTS_AVAIL(tp) \
        (tp->dirty_tx + NUM_TX_DESC - tp->cur_tx)

Both are also used in rtl_tx.

I don't get your point. Even a single variable is scattered through
the system.

-- 
Ueimor