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Date: Sun, 13 Jul 2008 02:28:13 +0200
From: Krzysztof Halasa <khc@...waw.pl>
To: <linux-kernel@...r.kernel.org>, netdev@...r.kernel.org
Subject: [bisected] kernel panic 2.6.22 -> 2.6.26-rc9+
Hi,
Bisected the panic. It seems it occurs on my ARM IXP4xx big-endian
system with USB ADSL 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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