[PATCH] tcp_splice: Don't wake up on input data if we can't write it anywhere

If we set the OUT_WAIT_* flag (waiting on EPOLLOUT) for a side of a given flow, it means that we're blocked, waiting for the receiver to actually receive data, with a full pipe. In that case, if we keep EPOLLIN set for the socket on the other side (our receiving side), we'll get into a loop such as: 41.0230: pasta: epoll event on connected spliced TCP socket 108 (events: 0x00000001) 41.0230: Flow 1 (TCP connection (spliced)): -1 from read-side call 41.0230: Flow 1 (TCP connection (spliced)): -1 from write-side call (passed 8192) 41.0230: Flow 1 (TCP connection (spliced)): event at tcp_splice_sock_handler:577 41.0230: pasta: epoll event on connected spliced TCP socket 108 (events: 0x00000001) 41.0230: Flow 1 (TCP connection (spliced)): -1 from read-side call 41.0230: Flow 1 (TCP connection (spliced)): -1 from write-side call (passed 8192) 41.0230: Flow 1 (TCP connection (spliced)): event at tcp_splice_sock_handler:577 leading to 100% CPU usage, of course. Drop EPOLLIN on our receiving side as long when we're waiting for output readiness on the other side. Link: https://github.com/containers/podman/issues/23686#issuecomment-2661036584 Link: https://www.reddit.com/r/podman/comments/1iph50j/pasta_high_cpu_on_podman_ro... Signed-off-by: Stefano Brivio --- tcp_splice.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/tcp_splice.c b/tcp_splice.c index f1a9223..8a39a6f 100644 --- a/tcp_splice.c +++ b/tcp_splice.c @@ -131,8 +131,12 @@ static void tcp_splice_conn_epoll_events(uint16_t events, ev[1].events = EPOLLOUT; } - flow_foreach_sidei(sidei) - ev[sidei].events |= (events & OUT_WAIT(sidei)) ? EPOLLOUT : 0; + flow_foreach_sidei(sidei) { + if (events & OUT_WAIT(sidei)) { + ev[sidei].events |= EPOLLOUT; + ev[!sidei].events &= ~EPOLLIN; + } + } } /** -- 2.43.0