netdev - Re: wrong smp_mb__after_atomic() in tcp_check

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Date:   Mon, 23 Jan 2017 11:56:18 -0500
From:   Jason Baron <jbaron@...mai.com>
To:     Oleg Nesterov <oleg@...hat.com>,
        "David S. Miller" <davem@...emloft.net>
Cc:     Herbert Xu <herbert.xu@...hat.com>,
        Yauheni Kaliuta <yauheni.kaliuta@...hat.com>,
        netdev@...r.kernel.org, Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: wrong smp_mb__after_atomic() in tcp_check_space() ?

On 01/23/2017 09:30 AM, Oleg Nesterov wrote:
> Hello,
>
> smp_mb__after_atomic() looks wrong and misleading, sock_reset_flag() does the
> non-atomic __clear_bit() and thus it can not guarantee test_bit(SOCK_NOSPACE)
> (non-atomic too) won't be reordered.
>

Indeed. Here's a bit of discussion on it:
http://marc.info/?l=linux-netdev&m=146662325920596&w=2

> It was added by 3c7151275c0c9a "tcp: add memory barriers to write space paths"
> and the patch looks correct in that we need the barriers in tcp_check_space()
> and tcp_poll() in theory, so it seems tcp_check_space() needs smp_mb() ?
>

Yes, I think it should be upgraded to an smp_mb() there. If you agree 
with this analysis, I will send a patch to upgrade it. Note, I did not 
actually run into this race in practice.

Thanks,

-Jason