netdev - wrong smp_mb__after_atomic() in tcp_check

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Date:   Mon, 23 Jan 2017 15:30:25 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Jason Baron <jbaron@...mai.com>,
        "David S. Miller" <davem@...emloft.net>
Cc:     Herbert Xu <herbert.xu@...hat.com>,
        Yauheni Kaliuta <yauheni.kaliuta@...hat.com>,
        netdev@...r.kernel.org
Subject: wrong smp_mb__after_atomic() in tcp_check_space() ?

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.

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() ?

Oleg.