netdev - Re: Soft lockup in inet_put

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Date:   Tue, 13 Dec 2016 15:32:09 -0800
From:   Tom Herbert <tom@...bertland.com>
To:     Craig Gallek <kraigatgoog@...il.com>
Cc:     Josef Bacik <jbacik@...com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: Soft lockup in inet_put_port on 4.6

On Tue, Dec 13, 2016 at 3:03 PM, Craig Gallek <kraigatgoog@...il.com> wrote:
> On Tue, Dec 13, 2016 at 3:51 PM, Tom Herbert <tom@...bertland.com> wrote:
>> I think there may be some suspicious code in inet_csk_get_port. At
>> tb_found there is:
>>
>>                 if (((tb->fastreuse > 0 && reuse) ||
>>                      (tb->fastreuseport > 0 &&
>>                       !rcu_access_pointer(sk->sk_reuseport_cb) &&
>>                       sk->sk_reuseport && uid_eq(tb->fastuid, uid))) &&
>>                     smallest_size == -1)
>>                         goto success;
>>                 if (inet_csk(sk)->icsk_af_ops->bind_conflict(sk, tb, true)) {
>>                         if ((reuse ||
>>                              (tb->fastreuseport > 0 &&
>>                               sk->sk_reuseport &&
>>                               !rcu_access_pointer(sk->sk_reuseport_cb) &&
>>                               uid_eq(tb->fastuid, uid))) &&
>>                             smallest_size != -1 && --attempts >= 0) {
>>                                 spin_unlock_bh(&head->lock);
>>                                 goto again;
>>                         }
>>                         goto fail_unlock;
>>                 }
>>
>> AFAICT there is redundancy in these two conditionals.  The same clause
>> is being checked in both: (tb->fastreuseport > 0 &&
>> !rcu_access_pointer(sk->sk_reuseport_cb) && sk->sk_reuseport &&
>> uid_eq(tb->fastuid, uid))) && smallest_size == -1. If this is true the
>> first conditional should be hit, goto done,  and the second will never
>> evaluate that part to true-- unless the sk is changed (do we need
>> READ_ONCE for sk->sk_reuseport_cb?).
> That's an interesting point... It looks like this function also
> changed in 4.6 from using a single local_bh_disable() at the beginning
> with several spin_lock(&head->lock) to exclusively
> spin_lock_bh(&head->lock) at each locking point.  Perhaps the full bh
> disable variant was preventing the timers in your stack trace from
> running interleaved with this function before?

Could be, although dropping the lock shouldn't be able to affect the
search state. TBH, I'm a little lost in reading function, the
SO_REUSEPORT handling is pretty complicated. For instance,
rcu_access_pointer(sk->sk_reuseport_cb) is checked three times in that
function and also in every call to inet_csk_bind_conflict. I wonder if
we can simply this under the assumption that SO_REUSEPORT is only
allowed if the port number (snum) is explicitly specified.

Tom