rcu_sched_state detected stalls on Alpha with generic config

[Michael Cree]

I am seeing "rcu_sched_state detected stall on CPU" messages on Alpha
architecture with a generic SMP config.  Interactive tasks are seen to
lock up, with "INFO: task X blocked for more than 120 seconds" in the
kernel logs, and eventual kernel oops and panic, on latest 3.2-rc4 and
traceable back to 3.0.  Bisection between 2.6.39 and 3.0 leads to commit:

09223371deac67d08ca0b70bd18787920284c967
rcu: Use softirq to address performance regression

as the first bad commit.

Tested on an Alpha ES45 (Titan) with three 1.25 GHz CPUs and 4 GByte
memory.  Testing procedure is to build git software and run its test
suite with -j4 in the make command argument.

The CPU stall messages and eventually system lockup is only seen with a
generic Alpha config, never with a Titan machine specific config.

An example of kernel logs is (this one probably produced when I tried to
shutdown the system when it is falling over):

[45360.930876] INFO: rcu_sched_state detected stall on CPU 1 (t=798848
jiffies)
[45360.931853] INFO: rcu_sched_state detected stalls on CPUs/tasks: { 1}
(detected by 0, t=798850 jiffies)
[45489.080225] INFO: task umount:17371 blocked for more than 120 seconds.
[45489.158350] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
disables this message.
[45489.252100] umount          D fffffc00013461ac     0 17371  17368
0x00000000
[45489.336084] fffffc00fdd53db8 fffffc00fdd97bb8 fffffc000108ca1c
fffffc00dcc9e800
[45489.422998]        fffffc00dcc9e810 fffffc00013b3a5d fffffc000106289c
fffffc00ff0dfda8
[45489.519678]        0000000000000000 fffffc000108c81c fffffc0001cd73f0
0000000000000001
[45489.615381]        fffffc00010627f0 0000000000000000 fffffc00dcc9e920
fffffc00ff0bf780
[45489.712060]        fffffc00010111b8 fffffc00ff0dfda8 fffffc00ff0dfde8
fffffc0001cdaa58
[45489.808740]        0000000000000000 0000000000000000 fffffc0000000000
fffffc0000000000
[45489.907373] Trace:
[45489.930810] [<fffffc000108ca1c>] watchdog+0x200/0x27c
[45489.991357] [<fffffc000106289c>] kthread+0xac/0xc4
[45490.048974] [<fffffc000108c81c>] watchdog+0x0/0x27c
[45490.107568] [<fffffc00010627f0>] kthread+0x0/0xc4
[45490.164209] [<fffffc00010111b8>] kernel_thread+0x28/0x90
[45490.227685]

Let me know if any other information is needed to narrow down the problem.

Cheers
Michael.
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