Re: [PATCH v2 8/8] tcp_splice: Improve EOF and read stall exit conditions

[Stefano Brivio <sbrivio@redhat.com>]

On Fri,  5 Jun 2026 10:34:16 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> At the end of our loop we have a conditional 'break' that exits if we're
> at EOF on the read side and have nothing left in the pipe.  This makes
> sense: at EOF there's nothing left to do read-side and with nothing in the
> pipe there's nothing to do write side either.
> 
> The same is true if the read side hit an EAGAIN and the pipe is empty:
> there's nothing we can do (for now) read side, and with an empty pipe
> nothing write side either.  So, generalise the condition to exit on either
> EOF or EAGAIN read side.
> 
> Furthermore, if the read side is at EOF or EAGAIN and there's already
> nothing in the pipe before the write-side splice(), then that write side
> splice() can't accomplish anything, so exit the loop early in that case
> avoiding a harmless but unnecessary write-splice().
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  tcp_splice.c | 20 +++++++++++++++-----
>  1 file changed, 15 insertions(+), 5 deletions(-)
> 
> Changes in v2:
>  * Duplicate rather than move the test, it's valuable in both places.
>  * Make comments and commit message clearer
> 
> diff --git a/tcp_splice.c b/tcp_splice.c
> index 565596d3..1e3c7749 100644
> --- a/tcp_splice.c
> +++ b/tcp_splice.c
> @@ -497,9 +497,17 @@ static int tcp_splice_forward(struct ctx *c,
>  
>  		flow_trace(conn, "%zi from read-side call", readlen);
>  
> -		if (!readlen) {
> -			conn_event(conn, FIN_RCVD(fromsidei));
> -		} else if (readlen > 0) {
> +		if (readlen <= 0) {
> +			if (!readlen) /* EOF */
> +				conn_event(conn, FIN_RCVD(fromsidei));
> +
> +			/* We're either blocked or at EOF on the read side, and
> +			 * there's nothing in the pipe so there's nothing to do
> +			 * write side either.
> +			 */
> +			if (!conn->pending[fromsidei])
> +				break;
> +		} else {
>  			conn->pending[fromsidei] += readlen;
>  
>  			if (readlen >= (long)c->tcp.pipe_size * 90 / 100)
> @@ -531,9 +539,11 @@ static int tcp_splice_forward(struct ctx *c,
>  
>  		conn->pending[fromsidei] -= written;
>  
> -		if (conn->events & FIN_RCVD(fromsidei) &&
> -		    !conn->pending[fromsidei])
> +		if (!conn->pending[fromsidei] && readlen <= 0) {
> +			/* Read side is EOF or EAGAIN, and we emptied the pipe.
> +			 * No more we can do for now, */

Changed "now," to "now.", added empty comment line, applied.

>  			break;
> +		}
>  	}
>  
>  	/* We need write-side wakeups if and only if we have data in the pipe to

-- 
Stefano