Subject: Re: [PATCH] tcp: splice: fix an infinite loop in tcp_read_sock()

From: Willy Tarreau <[email-masked]>
Date: Fri, 11 Jan 2013 00:01:45 +0100

Hi Eric,

On Thu, Jan 10, 2013 at 09:06:10AM -0800, Eric Dumazet wrote:
> From: Eric Dumazet <[email-masked]>
> 
> commit 02275a2ee7c0 (tcp: don't abort splice() after small transfers)
> added a regression.
> 
> 
> [   83.843570] INFO: rcu_sched self-detected stall on CPU
> [   83.844575] INFO: rcu_sched detected stalls on CPUs/tasks: { 6} (detected by 0, t=21002 jiffies, g=4457, c=4456, q=13132)
> [   83.844582] Task dump for CPU 6:
> [   83.844584] netperf         R  running task        0  8966   8952 0x0000000c
> [   83.844587]  0000000000000000 0000000000000006 0000000000006c6c 0000000000000000
> [   83.844589]  000000000000006c 0000000000000096 ffffffff819ce2bc ffffffffffffff10
> [   83.844592]  ffffffff81088679 0000000000000010 0000000000000246 ffff880c4b9ddcd8
> [   83.844594] Call Trace:
> [   83.844596]  [<ffffffff81088679>] ? vprintk_emit+0x1c9/0x4c0
> [   83.844601]  [<ffffffff815ad449>] ? schedule+0x29/0x70
> [   83.844606]  [<ffffffff81537bd2>] ? tcp_splice_data_recv+0x42/0x50
> [   83.844610]  [<ffffffff8153beaa>] ? tcp_read_sock+0xda/0x260
> [   83.844613]  [<ffffffff81537b90>] ? tcp_prequeue_process+0xb0/0xb0
> [   83.844615]  [<ffffffff8153c0f0>] ? tcp_splice_read+0xc0/0x250
> [   83.844618]  [<ffffffff814dc0c2>] ? sock_splice_read+0x22/0x30
> [   83.844622]  [<ffffffff811b820b>] ? do_splice_to+0x7b/0xa0
> [   83.844627]  [<ffffffff811ba4bc>] ? sys_splice+0x59c/0x5d0
> [   83.844630]  [<ffffffff8119745b>] ? putname+0x2b/0x40
> [   83.844633]  [<ffffffff8118bcb4>] ? do_sys_open+0x174/0x1e0
> [   83.844636]  [<ffffffff815b6202>] ? system_call_fastpath+0x16/0x1b
> 
> 
> if recv_actor() returns 0, we should stop immediately,
> because looping wont give a chance to drain the pipe.
> 
> Signed-off-by: Eric Dumazet <[email-masked]>
> Cc: Willy Tarreau <[email-masked]>
> ---
>  net/ipv4/tcp.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 1ca2536..5f173dc 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -1482,7 +1482,7 @@ int tcp_read_sock(struct sock *sk, read_descriptor_t *desc,
>  					break;
>  			}
>  			used = recv_actor(desc, skb, offset, len);
> -			if (used < 0) {
> +			if (used <= 0) {
>  				if (!copied)
>  					copied = used;
>  				break;

Thanks for catching this one. I'm amazed we didn't notice it earlier,
I've been running my stress-test kernels with this patch applied since
we did it.

Willy