update: in the past 5 days of -tip testing i've gathered about 10
randconfig kernel configs that all produced such failures.
Since the bug itself is very elusive (it takes up to 50 boot +
kernel-rebuild-via-distccc iterations to trigger) bisection was still
not an option - but with 10 configs statistical analysis of the configs
is now possible.
I made a histogram of all kernel options present in those configs, and
one networking related kernel option stood out:
5 CONFIG_TCP_CONG_ADVANCED=y
6 CONFIG_INET_TCP_DIAG=y
6 CONFIG_TCP_MD5SIG=y
9 CONFIG_TCP_CONG_CUBIC=y
that code is called in the bootlogs:
the likelyhood of CONFIG_TCP_CONG_CUBIC=y being enabled in my randconfig
runs is 75%. The likelyhood of CONFIG_TCP_CONG_CUBIC=y being enabled in
10 configs in a row is 0.75^10, or 5.6%. So statistical analysis can say
it with a 95% confidence that the presence of this option correlates to
the hung sockets.
i have started testing this theory now, via the patch below, which turns
off TCP_CONG_CUBIC. It will take about 50 bootups on the affected
testsystems to confirm. (it will take a couple of hours today as not all
testsystems show these hung socket symptoms)
distributions enable TCP_CONG_CUBIC by default:
$ grep CUBIC /boot/config-2.6.24.7-92.fc8
CONFIG_TCP_CONG_CUBIC=y
CONFIG_DEFAULT_CUBIC=y
which would explain why Arjan and Peter triggered similar hangs as well.
Ingo
---------------------->
Subject: qa: no TCP_CONG_CUBIC
From: Ingo Molnar <mingo@elte.hu>
Date: Thu May 29 09:45:51 CEST 2008
---
net/ipv4/Kconfig | 4 ++++
1 file changed, 4 insertions(+)
Index: tip/net/ipv4/Kconfig
===================================================================
--- tip.orig/net/ipv4/Kconfig
+++ tip/net/ipv4/Kconfig
@@ -454,6 +454,8 @@ config TCP_CONG_BIC
config TCP_CONG_CUBIC
tristate "CUBIC TCP"
default y
+ depends on BROKEN_BOOT_ALLOWED
+ select BROKEN_BOOT
---help---
This is version 2.0 of BIC-TCP which uses a cubic growth function
among other techniques.
@@ -608,6 +610,8 @@ endif
config TCP_CONG_CUBIC
tristate
depends on !TCP_CONG_ADVANCED
+ depends on BROKEN_BOOT_ALLOWED
+ select BROKEN_BOOT
default y
config DEFAULT_TCP_CONG
--
