Re: [PATCH v7 1/3] tcp: move seq_to_tap update to when frame is queued

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, May 24, 2024 at 01:26:54PM -0400, Jon Maloy wrote:
> commit a469fc393fa1 ("tcp, tap: Don't increase tap-side sequence counter for dropped frames")
> delayed update of conn->seq_to_tap until the moment the corresponding
> frame has been successfully pushed out. This has the advantage that we
> immediately can make a new attempt to transmit a frame after a failed
> trasnmit, rather than waiting for the peer to later discover a gap and
> trigger the fast retransmit mechanism to solve the problem.
> 
> This approach has turned out to cause a problem with spurious sequence
> number updates during peer-initiated retransmits, and we have realized
> it may not be the best way to solve the above issue.
> 
> We now restore the previous method, by updating the said field at the
> moment a frame is added to the outqueue. To retain the advantage of
> having a quick re-attempt based on local failure detection, we now scan
> through the part of the outqueue that had do be dropped, and restore the
> sequence counter for each affected connection to the most appropriate
> value.
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>

This still has the issues I pointed out on the last revision...

[snip]
> +/**
> + * tcp_revert_seq() - Revert affected conn->seq_to_tap after failed transmission
> + * @conns:       Array of connection pointers corresponding to queued frames
> + * @frames:      Two-dimensional array containing queued frames with sub-iovs
> + * @num_frames:  Number of entries in the two arrays to be compared
> + */
> +static void tcp_revert_seq(struct tcp_tap_conn **conns, struct iovec (*frames)[TCP_NUM_IOVS],
> +			   int num_frames)
> +{
> +	int i;
> +
> +	for (i = 0; i < num_frames; i++) {
> +		struct tcp_tap_conn *conn = conns[i];
> +		struct tcphdr *th = frames[i][TCP_IOV_PAYLOAD].iov_base;
> +		uint32_t seq = ntohl(th->seq);
> +
> +		if (SEQ_LE(conn->seq_to_tap, seq))
> +			continue;
> +
> +		conn->seq_to_tap = seq;

...one trivial - this would be clearer without the continue - ...

> +	}
> +}
> +
>  /**
>   * tcp_payload_flush() - Send out buffers for segments with data
>   * @c:		Execution context
>   */
>  static void tcp_payload_flush(const struct ctx *c)
>  {
> -	unsigned i;
>  	size_t m;
>  
>  	m = tap_send_frames(c, &tcp6_l2_iov[0][0], TCP_NUM_IOVS,
>  			    tcp6_payload_used);
> -	for (i = 0; i < m; i++)
> -		*tcp6_seq_update[i].seq += tcp6_seq_update[i].len;
> +	if (m != tcp6_payload_used) {
> +		tcp_revert_seq(tcp6_frame_conns, &tcp6_l2_iov[m],
> +			       tcp6_payload_used - m);

.. and one fatal - you're calling this with non-matching entries from
frame_conns[] and l2_iov[].

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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