netdev - Re: [PATCH net 3/3] tcp: fix xmit timer to only be reset if data ACKed/SACKed

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Date:   Tue, 1 Aug 2017 10:24:30 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     maowenan <maowenan@...wei.com>
Cc:     David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Yuchung Cheng <ycheng@...gle.com>,
        Nandita Dukkipati <nanditad@...gle.com>
Subject: Re: [PATCH net 3/3] tcp: fix xmit timer to only be reset if data ACKed/SACKed

On Tue, Aug 1, 2017 at 8:20 AM, maowenan <maowenan@...wei.com> wrote:
> > +     /* If needed, reset TLP/RTO timer; RACK may later override this. */
> [Mao Wenan] I have question about RACK, if there is no RACK feature
> in lower version, who can clear this flag:FLAG_SET_XMIT_TIMER?

In the comment, "this" is referring to the xmit timer. The comment is
referring to the fact that RACK (or similarly
tcp_pause_early_retransmit() in earlier kernels) can override the
value of the xmit timer set in tcp_set_xmit_timer(). The comment is
not claiming that RACK clears the FLAG_SET_XMIT_TIMER bit in "flag".

Note that "flag" is an argument to tcp_ack(). And while processing a
given incoming ACK, the callers of tcp_ack() pass in either 0 or one
or more constant bit flags. So FLAG_SET_XMIT_TIMER and all the other
bit flags are implicitly set to zero at the beginning of processing
each ACK.

neal