Re: [PATCH net-next 01/16] mptcp: use WRITE_ONCE/READ_ONCE for the pernet *_max

[Jakub Kicinski]

On Thu, 28 Jan 2021 17:11:00 -0800 Mat Martineau wrote:
> +     if (entry->addr.flags & MPTCP_PM_ADDR_FLAG_SIGNAL) {
> +             addr_max = READ_ONCE(pernet->add_addr_signal_max);
> +             WRITE_ONCE(pernet->add_addr_signal_max, addr_max + 1);
> +     }

This is an odd construct.

READ_ONCE() is used when the value can change underneath the reader,
not in writers. If we want to increment a variable, there must either
be a writer side lock, or the variable has to be switched to atomic_t.

I'm guessing the former is the case here, so there can be no concurrent
writers. Please omit the READ_ONCE():

        if (entry->addr.flags & MPTCP_PM_ADDR_FLAG_SIGNAL)
                WRITE_ONCE(pernet->add_addr_signal_max, 
                           pernet->add_addr_signal_max + 1);


Same for other 3 cases.