Subject: Re: [PATCH] tcp: set SPLICE_F_NONBLOCK after first buffer has been spliced

From: Eric Dumazet <[email-masked]>
Date: Thu, 05 Nov 2009 11:30:01 +0100

Max Kellermann a écrit :
> When splicing a large amount of bytes from a TCP socket to a pipe
> (more than PIPE_BUFFERS), splice() can block, even though the pipe was
> empty.  The correct behavior would be to copy as much as possible, and
> return without blocking.  Block only if nothing can be transferred.
> When the destination pipe is (initially) writable, splice() should do
> the same with or without SPLICE_F_NONBLOCK.
> 
> The cause is the loop in tcp_splice_read(): it calls
> __tcp_splice_read() (and thus skb_splice_bits() and splice_to_pipe())
> again and again, until the requested number of bytes has been
> transferred, or an error occurs.  In the first iteration, up to 64 kB
> is copied, and the second iteration will block, because
> splice_to_pipe() is called again and sees the pipe is already full.
> 
> This patch adds SPLICE_F_NONBLOCK to the splice flags after the first
> iteration has finished successfully.  This prevents the second
> splice_to_pipe() call from blocking.  The resulting EAGAIN error is
> handled gracefully, and tcp_splice_read() returns the number of bytes
> successfully moved.
> ---
> 
>  net/ipv4/tcp.c |    5 +++++
>  1 files changed, 5 insertions(+), 0 deletions(-)
> 
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 9114524..0f8b01f 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -628,6 +628,11 @@ ssize_t tcp_splice_read(struct socket *sock, loff_t *ppos,
>  		    (sk->sk_shutdown & RCV_SHUTDOWN) ||
>  		    signal_pending(current))
>  			break;
> +
> +		/* the following splice_to_pipe() calls should not
> +		   block, because we have already successfully
> +		   transferred at least one buffer */
> +		tss.flags |= SPLICE_F_NONBLOCK;
>  	}
>  
>  	release_sock(sk);
> 

CC netdev

I dont think this patch is correct. Could you describe your use case ?

If you dont want to block on output pipe, you should set this NONBLOCK 
flag before calling splice(SPLICE_F_NONBLOCK) syscall.

ie : Use the socket in blocking mode, but output pipe in non-blocking mode.

Some application could have a thread working in full blocking mode, and have another
thread reading the pipe (and eventually unblocking first thread)

Thanks