From b64ef531b08a2969e26a2212499734940a0c6335 Mon Sep 17 00:00:00 2001 From: David Gibson Date: Mon, 18 May 2026 13:22:43 +1000 Subject: conf, repair, tap: Document reasons for blocking Unix sockets Most of our operation is asynchronous, based on non-blocking fds handled in our epoll loop. However, our several Unix sockets (tap client, repair helper, control client) are all blocking fds after accept(). That is in fact correct, but for not especially obvious reasons that are slightly different in each case. Add explanatory comments to each of them. Signed-off-by: David Gibson [sbrivio: Fixed minor coding style detail in comment in conf_accept()] Signed-off-by: Stefano Brivio --- conf.c | 7 +++++++ repair.c | 4 ++++ tap.c | 5 +++++ 3 files changed, 16 insertions(+) diff --git a/conf.c b/conf.c index 029b9c7..6f86940 100644 --- a/conf.c +++ b/conf.c @@ -2084,6 +2084,13 @@ static void conf_accept(struct ctx *c) int fd, rc; retry: + /* Currently we perform the configuration transaction more-or-less + * synchronously, so we want the accepted socket to be blocking. + * + * FIXME: We should make the configuration update asynchronous, like + * most of our operation, so a misbehaving configuration client can't + * block the main forwarding loop. + */ fd = accept4(c->fd_control_listen, NULL, NULL, SOCK_CLOEXEC); if (fd < 0) { if (errno != EAGAIN) diff --git a/repair.c b/repair.c index 3e0e3e0..f31ccce 100644 --- a/repair.c +++ b/repair.c @@ -99,6 +99,10 @@ int repair_listen_handler(struct ctx *c, uint32_t events) return EEXIST; } + /* We want the accepted socket to be blocking; we use it during + * migration which is a synchronous interruption to our normal + * non-blocking behaviour. + */ if ((c->fd_repair = accept4(c->fd_repair_listen, NULL, NULL, SOCK_CLOEXEC)) < 0) { rc = errno; diff --git a/tap.c b/tap.c index b318327..4cba4c7 100644 --- a/tap.c +++ b/tap.c @@ -1492,6 +1492,11 @@ void tap_listen_handler(struct ctx *c, uint32_t events) return; } + /* Because we generally only access the accepted socket from epoll + * events, it usually doesn't matter if it's blocking or non-blocking. + * However, in rare cases when the socket buffer fills we need (briefly, + * we hope) blocking writes (write_remainder() in send_frames_passt()). + */ c->fd_tap = accept4(c->fd_tap_listen, NULL, NULL, SOCK_CLOEXEC); if (c->fd_tap < 0) { warn_perror("Error accepting tap client"); -- cgit v1.2.3