linux-kernel - Re: [PATCH 4.19 625/639] packet: fix data-race in fanout_flow_is

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Date:   Sun, 26 Jan 2020 10:38:41 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Pavel Machek <pavel@...x.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org,
        Willem de Bruijn <willemb@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 4.19 625/639] packet: fix data-race in fanout_flow_is_huge()

On Sun, Jan 26, 2020 at 10:29 AM Pavel Machek <pavel@...x.de> wrote:
>
>
> > -     po->rollover->history[prandom_u32() % ROLLOVER_HLEN] = rxhash;
> > +     victim = prandom_u32() % ROLLOVER_HLEN;
> > +
> > +     /* Avoid dirtying the cache line if possible */
> > +     if (READ_ONCE(history[victim]) != rxhash)
> > +             WRITE_ONCE(history[victim], rxhash);
> > +
>
> Replacing simple asignment with if() is ... not nice and with all the
> "volatile" magic in _ONCE macros may not be win for
> everyone. [Actually, I don't think this is win here. This is not
> exactly hot path, is it?


This is a critical hot path, eg under DDOS attack.

>
> If this is going to get more common, should we get
> WRITE_ONCE_NONDIRTY() macro hiding the uglyness?
>

Sure, but we do not add macros for stable patches, usually.

Honestly, WRITE_ONCE_NONDIRTY() looks ugly to me.

Note that the KCSAN race might be solved in another way when
data_race() is available.