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Date: Sun, 13 Jul 2008 01:31:07 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Krzysztof Halasa <khc@...waw.pl>
Cc: <linux-kernel@...r.kernel.org>, netdev@...r.kernel.org,
Stephen Hemminger <shemminger@...ux-foundation.org>,
"David S. Miller" <davem@...emloft.net>
Subject: Re: [bisected] kernel panic 2.6.22 -> 2.6.26-rc9+
(cc's added)
On Sun, 13 Jul 2008 02:28:13 +0200 Krzysztof Halasa <khc@...waw.pl> wrote:
> Hi,
>
> Bisected the panic. It seems it occurs on my ARM IXP4xx big-endian
> system with USB ADS
I guess you're referring to this:
http://linux.derkeiler.com/Mailing-Lists/Kernel/2008-07/msg04754.html
> PPP over ATM connection (Thomson/Alcatel
> Speedtouch). It doesn't seem to occur on the same IXP4xx with Ethernet
> or V.35 WAN, and it doesn't occur on i386 + the same Speedtouch ADSL.
> The kernel is basically unpatched, the only extra patch applied is the
> platform support (nothing magic).
>
> Generally to trigger the panic one has to request a TCP data stream
> over that PPPoATM connection.
>
> Reverting the commit on top of 2.6.25.10 fixes the problem.
>
> Now what's wrong with that?
>
> 7e58886b45bc4a309aeaa8178ef89ff767daaf7f:
> author Stephen Hemminger <shemminger@...ux-foundation.org>
> [TCP]: cubic optimization
>
> Use willy's work in optimizing cube root by having table for small values.
>
> --- a/net/ipv4/tcp_cubic.c
> +++ b/net/ipv4/tcp_cubic.c
> @@ -91,23 +91,51 @@ static void bictcp_init(struct sock *sk)
> tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
> }
>
> -/*
> - * calculate the cubic root of x using Newton-Raphson
> +/* calculate the cubic root of x using a table lookup followed by one
> + * Newton-Raphson iteration.
> + * Avg err ~= 0.195%
> */
> static u32 cubic_root(u64 a)
> {
> - u32 x;
> -
> - /* Initial estimate is based on:
> - * cbrt(x) = exp(log(x) / 3)
> + u32 x, b, shift;
> + /*
> + * cbrt(x) MSB values for x MSB values in [0..63].
> + * Precomputed then refined by hand - Willy Tarreau
> + *
> + * For x in [0..63],
> + * v = cbrt(x << 18) - 1
> + * cbrt(x) = (v[x] + 10) >> 6
> */
> - x = 1u << (fls64(a)/3);
> + static const u8 v[] = {
> + /* 0x00 */ 0, 54, 54, 54, 118, 118, 118, 118,
> + /* 0x08 */ 123, 129, 134, 138, 143, 147, 151, 156,
> + /* 0x10 */ 157, 161, 164, 168, 170, 173, 176, 179,
> + /* 0x18 */ 181, 185, 187, 190, 192, 194, 197, 199,
> + /* 0x20 */ 200, 202, 204, 206, 209, 211, 213, 215,
> + /* 0x28 */ 217, 219, 221, 222, 224, 225, 227, 229,
> + /* 0x30 */ 231, 232, 234, 236, 237, 239, 240, 242,
> + /* 0x38 */ 244, 245, 246, 248, 250, 251, 252, 254,
> + };
> +
> + b = fls64(a);
> + if (b < 7) {
> + /* a in [0..63] */
> + return ((u32)v[(u32)a] + 35) >> 6;
> + }
> +
> + b = ((b * 84) >> 8) - 1;
> + shift = (a >> (b * 3));
>
> - /* converges to 32 bits in 3 iterations */
> - x = (2 * x + (u32)div64_64(a, (u64)x*(u64)x)) / 3;
> - x = (2 * x + (u32)div64_64(a, (u64)x*(u64)x)) / 3;
> - x = (2 * x + (u32)div64_64(a, (u64)x*(u64)x)) / 3;
> + x = ((u32)(((u32)v[shift] + 10) << b)) >> 6;
>
> + /*
> + * Newton-Raphson iteration
> + * 2
> + * x = ( 2 * x + a / x ) / 3
> + * k+1 k k
> + */
> + x = (2 * x + (u32)div64_64(a, (u64)x * (u64)(x - 1)));
> + x = ((x * 341) >> 10);
> return x;
> }
>
> PC is at bictcp_cong_avoid+0x33c/0x454
>
> Backtrace:
>
> bictcp_cong_avoid+0x0/0x454 from tcp_cong_avoid+0x1c/0x30
> tcp_cong_avoid+0x0/0x30 from tcp_ack+0x156c/0x1c6c
> r4:00000000
> tcp_ack+0x0/0x1c6c from tcp_rcv_established+0x4a8/0x76c
> tcp_rcv_established+0x0/0x76c from tcp_v4_do_rcv+0x25c/0x458
> tcp_v4_do_rcv+0x0/0x458 from tcp_v4_rcv+0x6c4/0x7ec
> tcp_v4_rcv+0x0/0x7ec from ip_local_deliver_finish+0xe4/0x24c
> ip_local_deliver_finish+0x0/0x24c from ip_local_deliver+0x4c/0xac
> r7:c043817c r6:c6818612 r5:c6a6a3c0 r4:c6a6a3c0
> ip_local_deliver+0x0/0xac from ip_rcv_finish+0x114/0x378
> r4:80000000
> ip_rcv_finish+0x0/0x378 from ip_rcv+0x1f8/0x2a8
> ip_rcv+0x0/0x2a8 from netif_receive_skb+0x36c/0x4a4
> r7:00000800 r6:c0405e80 r5:c6a6a3c0 r4:c0436d54
> netif_receive_skb+0x0/0x4a4 from process_backlog+0x88/0x14c
> r8:00000040 r7:c0436d24 r6:00000000 r5:c0436d0c r4:c6912c00
> process_backlog+0x0/0x14c from net_rx_action+0xb4/0x1c4
> net_rx_action+0x0/0x1c4 from __do_softirq+0x68/0xe0
> _do_softirq+0x0/0xe0 from irq_exit+0x48/0x50
> r7:00000000 r6:c04344d0 r5:c040ae24 r4:00000014
> irq_exit+0x0/0x50 from asm_do_IRQ+0x48/0x5c
> asm_do_IRQ+0x0/0x5c from __irq_svc+0x24/0x60
> ...
> --
> Krzysztof Halasa
> --
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