Re: [PATCH v2 32/48] multipathd: uxlsnr: use poll loop for sending, too

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On Thu, Nov 18, 2021 at 11:58:24PM +0100, 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.
> 

The last time I reviewed this, I mentioned that when we fall through
from CLT_WORK to CLT_SEND, the client hasn't checked for a POLLOUT
revent, so it's possible to block here. I'm not sure that we care.
If not

Reviewed-by: Benjamin Marzinski <bmarzins@xxxxxxxxxx>

> Signed-off-by: Martin Wilck <mwilck@xxxxxxxx>
> ---
>  multipathd/uxlsnr.c | 35 ++++++++++++++++++++++++++---------
>  1 file changed, 26 insertions(+), 9 deletions(-)
> 
> diff --git a/multipathd/uxlsnr.c b/multipathd/uxlsnr.c
> index 45fe7b5..6213454 100644
> --- a/multipathd/uxlsnr.c
> +++ b/multipathd/uxlsnr.c
> @@ -447,7 +447,6 @@ static int client_state_machine(struct client *c, struct vectors *vecs,
>  				short revents)
>  {
>  	ssize_t n;
> -	const char *buf;
>  
>  	condlog(4, "%s: cli[%d] poll=%x state=%d cmd=\"%s\" repl \"%s\"", __func__,
>  		c->fd, revents, c->state, c->cmd, get_strbuf_str(&c->reply));
> @@ -544,16 +543,31 @@ static int client_state_machine(struct client *c, struct vectors *vecs,
>  		if (get_strbuf_len(&c->reply) == 0)
>  			default_reply(c, c->error);
>  
> -		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))
> +				c->error = -ECONNRESET;
> +			c->cmd_len = len;
> +			return STM_BREAK;
> +		}
>  
> -		set_client_state(c, CLT_RECV);
> +		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))
> +					c->error = -ECONNRESET;
> +			} else
> +				c->len += n;
> +		}
> +
> +                if (c->len >= c->cmd_len) {
> +			condlog(4, "cli[%d]: Reply [%zu bytes]", c->fd, c->cmd_len);
> +			set_client_state(c, CLT_RECV);
> +		}
>  		return STM_BREAK;
>  
>  	default:
> @@ -686,6 +700,9 @@ 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;
> -- 
> 2.33.1

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