Re: [PATCH net 1/6] sctp: remove the unnecessary state check in sctp_outq_tail

On Fri, Sep 09, 2016 at 03:11:57PM +0800, Xin Long wrote:

quoted

quoted

Its not enough to just look at the paths where outq_tail is called, because
the outq_tail function is checking a state variable that can update
asynchronously in the side effect processing queue.  Look at any one of the
timer functions.  Those all fire asynchronous to the outq list, but they all

Fortunately, sctp_do_sm is protected by lock_sock.
I just checked all the sctp_do_sm calling in  timer functions.

------------
        bh_lock_sock(sk);
        if (sock_owned_by_user(sk)) {    <----------- [1]
                pr_debug("%s: sock is busy\n", __func__);
                /* Try again later.  */
                mod_timer()
                goto out_unlock;
        }
        error = sctp_do_sm(net, SCTP_EVENT_T_TIMEOUT,
out_unlock:
        bh_unlock_sock(sk);


which means sctp_do_sm is *non-reenterable* because of
sock lock's protection. even if it's interrupted by something
like timers, they have to delay their real process until sock
lock is released.

So It's still safe here. pls double-check.

I don't know, I still don't feel safe about it.  I agree the socket lock keeps
the state from changing during a single transmission, which makes the use case
you are focused on correct.

That said, have you considered the retransmit case?  That is to say, if you
queue and flush the outq, and some packets fail delivery, and in the time
between the intial send and the expiration of the RTX timer (during which the
socket lock will have been released), an event may occur which changes the
transport state, which will then be ignored with your patch.

Neil