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

[Stefano Brivio <sbrivio@redhat.com>]

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_rootless_container/
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
 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;
+		}
+	}
 }
 
 /**
-- 
@@ -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