Re: [PATCH 10/11] tcp_splice: Exploit side symmetry in tcp_splice_destroy()

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

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On Fri, Nov 03, 2023 at 05:22:13PM +0100, Stefano Brivio wrote:
> On Thu, 12 Oct 2023 12:51:13 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > tcp_splice_destroy() has some close-to-duplicated logic handling closing of
> > the socket and ipies for each side of the connection.  We can use a loop
>                  ^^^^^ pipes

Oops, fixed.

> > across the sides to reduce the duplication.
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  tcp_splice.c | 32 ++++++++++++++------------------
> >  1 file changed, 14 insertions(+), 18 deletions(-)
> > 
> > diff --git a/tcp_splice.c b/tcp_splice.c
> > index 99ef8a4..239f6d2 100644
> > --- a/tcp_splice.c
> > +++ b/tcp_splice.c
> > @@ -258,30 +258,26 @@ void tcp_splice_conn_update(const struct ctx *c, struct tcp_splice_conn *new)
> >  void tcp_splice_destroy(struct ctx *c, union tcp_conn *conn_union)
> >  {
> >  	struct tcp_splice_conn *conn = &conn_union->splice;
> > +	int side;
> >  
> > -	if (conn->events & SPLICE_ESTABLISHED) {
> > -		/* Flushing might need to block: don't recycle them. */
> > -		if (conn->pipe[0][0] != -1) {
> > -			close(conn->pipe[0][0]);
> > -			close(conn->pipe[0][1]);
> > -			conn->pipe[0][0] = conn->pipe[0][1] = -1;
> > +	for (side = 0; side < SIDES; side++) {
> > +		if (conn->events & SPLICE_ESTABLISHED) {
> > +			/* Flushing might need to block: don't recycle them. */
> > +			if (conn->pipe[side][0] != -1) {
> > +				close(conn->pipe[side][0]);
> > +				close(conn->pipe[side][1]);
> > +				conn->pipe[side][0] = conn->pipe[side][1] = -1;
> > +			}
> >  		}
> > -		if (conn->pipe[1][0] != -1) {
> > -			close(conn->pipe[1][0]);
> > -			close(conn->pipe[1][1]);
> > -			conn->pipe[1][0] = conn->pipe[1][1] = -1;
> > +
> > +		if (side == 0 || conn->events & SPLICE_CONNECT) {
> > +			close(conn->s[side]);
> > +			conn->s[side] = -1;
> >  		}
> > -	}
> >  
> > -	if (conn->events & SPLICE_CONNECT) {
> > -		close(conn->s[1]);
> > -		conn->s[1] = -1;
> > +		conn->read[side] = conn->written[side] = 0;
> >  	}
> >  
> > -	close(conn->s[0]);
> > -	conn->s[0] = -1;
> > -	conn->read[0] = conn->written[0] = conn->read[1] = conn->written[1] = 0;
> 
> With this, on SPLICE_CONNECT, we would close the [0] side, but not the
> [1] side. SPLICE_CONNECT means we already have an open socket for [1],
> though. I think it should be:
> 
> 		[loop on sides]
> 
> 		if (side == 1 || conn->events & SPLICE_CONNECT) {
> 			close(conn->s[side]);
> 			conn->s[1] = -1;
> 		}
> 	}
> 
> and then we still need to unconditionally close conn->s[0]. Perhaps we could
> take both parts outside of the loop:

Uh.. I think you're misreading.  In the updated code we have:
		if (side == 0 || conn->events & SPLICE_CONNECT) {
			close(conn->s[side]);
			conn->s[side] = -1;
		}
That's an OR, so we always close side 0, and we close side 1 iff we
have SPLICE_CONNECT, which matches what you're describing.

> 	if (conn->events & SPLICE_CONNECT) {
> 		close(conn->s[1]);
> 		conn->s[1] = -1;
> 	}
> 
> 	close(conn->s[0]);
> 	conn->s[0] = -1;
> 	conn->read[side] = conn->written[side] = 0;
> 
> The handling for the SPLICE_ESTABLISHED case looks correct to me.

We are relying on the fact that setting SPLICE_ESTABLISHED doesn't
clear SPLICE_CONNECT.

-- 
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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