On Fri, Sep 10, 2021 at 01:41:18PM +0200, mwilck@xxxxxxxx wrote:
> From: Martin Wilck <mwilck@xxxxxxxx>
>
> send_packet() may busy-loop. By polling for POLLOUT, we can
> avoid that, even if it's very unlikely in practice.
>
> Signed-off-by: Martin Wilck <mwilck@xxxxxxxx>
> ---
> multipathd/uxlsnr.c | 39 ++++++++++++++++++++++++++++++++-------
> 1 file changed, 32 insertions(+), 7 deletions(-)
>
> diff --git a/multipathd/uxlsnr.c b/multipathd/uxlsnr.c
> index 1bf4126..c18b2c4 100644
> --- a/multipathd/uxlsnr.c
> +++ b/multipathd/uxlsnr.c
> @@ -588,15 +588,37 @@ static void handle_client(struct client *c, struct vectors *vecs, short revents)
> if (get_strbuf_len(&c->reply) == 0)
> default_reply(c, c->error);
>
> - const char *buf = get_strbuf_str(&c->reply);
> + if (c->cmd_len == 0) {
> + size_t len = get_strbuf_len(&c->reply) + 1;
>
> - if (send_packet(c->fd, buf) != 0)
> - dead_client(c);
> - else
> - condlog(4, "cli[%d]: Reply [%zu bytes]", c->fd,
> - get_strbuf_len(&c->reply) + 1);
> - reset_strbuf(&c->reply);
> + if (send(c->fd, &len, sizeof(len), MSG_NOSIGNAL)
> + != sizeof(len)) {
This assumes that failing to send the size is always an error. What
about if we get EINTR/EAGAIN? Also, it seems pretty likely that we will
either send all of the size or none of it, but I'm not sure we can
guarantee that. send_packet() handled partitial writes of the length.
Actually, mpath_recv_reply_len() which is still used by CLT_RECV still
uses read_all(), instead of just polling again on partial reads.
Also, the fd is not set to be non-blocking. and if we fall through to
CLT_SEND, we haven't checked for a POLLOUT revent, so technically, I
believe the write could block here.
-Ben
> + c->error = -ECONNRESET;
> + return;
> + }
> + c->cmd_len = len;
> + return;
> + }
>
> + if (c->len < c->cmd_len) {
> + const char *buf = get_strbuf_str(&c->reply);
> +
> + n = send(c->fd, buf + c->len, c->cmd_len, MSG_NOSIGNAL);
> + if (n == -1) {
> + if (errno == EAGAIN || errno == EINTR)
> + return;
> + else {
> + c->error = -ECONNRESET;
> + return;
> + }
> + }
> + c->len += n;
> + }
> + if (c->len < c->cmd_len)
> + /* continue polling */
> + return;
> +
> + condlog(4, "cli[%d]: Reply [%zu bytes]", c->fd, c->cmd_len);
> set_client_state(c, CLT_RECV);
> break;
>
> @@ -701,6 +723,9 @@ void *uxsock_listen(long ux_sock, void *trigger_data)
> case CLT_RECV:
> polls[i].events = POLLIN;
> break;
> + case CLT_SEND:
> + polls[i].events = POLLOUT;
> + break;
> default:
> /* don't poll for this client */
> continue;
> --
> 2.33.0
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