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Subjectpreempt-related hangs
I sent this email to Ingo last week; seems that he's
having some downtime. It was happening on my dual PIII
and I now discover that the quad pIII does the same
thing. Any ideas?



Kernel is 2.5.7, dual PIII. When I enable preempt it
locks during boot.

I applied the kgdb patch and had a poke.

(gdb) info threads
* 6 Thread 6 preempt_schedule () at sched.c:848
5 Thread 5 preempt_schedule () at sched.c:848
4 Thread 4 context_thread (startup=0xc0395f90) at context.c:101
3 Thread 3 migration_thread (unused=0x0) at sched.c:1646
2 Thread 2 migration_thread (unused=0x0) at sched.c:1646
1 Thread 1 spawn_ksoftirqd () at softirq.c:407

Note that init is stuck in spawn_ksoftirqd. It's spinning in
that function, yielding, waiting for the softirqd threads to
come alive. They're threads 5 and 6.

(gdb) thread 6
[Switching to thread 6 (Thread 6)]#0 preempt_schedule () at sched.c:848
848 }
(gdb) bt
#0 preempt_schedule () at sched.c:848
#1 0xc0117f77 in try_to_wake_up (p=0xefef4dc0) at sched.c:179
#2 0xc0117f8d in wake_up_process (p=0xefef68c0) at sched.c:347
#3 0xc011ad70 in set_cpus_allowed (p=0xefef68c0, new_mask=2) at sched.c:1583
#4 0xc01247fe in ksoftirqd (__bind_cpu=0x1) at softirq.c:371
#5 0xc010586b in kernel_thread (fn=0xc038aa97 <spawn_ksoftirqd+71>, arg=0x10, flags=582)
at process.c:501
#6 0xffffffff in ?? ()


So ksoftirqd has entered set_cpus_allowed(), and has tried to wake
migration_thread.

try_to_wake_up() has called task_rq_unlock(rq, &flags); and
task_rq_unlock() has done preempt_enable(). Game over at that
point. Looks like ksoftirqd has scheduled away on that preempt_enable
and is never coming back.

(gdb) info registers
eax 0xefef68c0 -269522752
ecx 0xc03d1540 -1069738688
edx 0x0 0
ebx 0xefed8000 -269647872
esp 0xefed9f18 0xefed9f18
...
(gdb) p *((struct thread_info *)0xefed8000)->task
$8 = {state = 0, thread_info = 0xefed8000, usage = {counter = 1}, flags = 64,
ptrace = 0, lock_depth = -1, prio = 139, static_prio = 139, run_list = {
next = 0xc03d239c, prev = 0xc03d239c}, array = 0xc03d1f2c, sleep_avg = 0,
sleep_timestamp = 117, policy = 0, cpus_allowed = 2, time_slice = 8, tasks = {
next = 0xc0360580, prev = 0xefef6240}, mm = 0x0, active_mm = 0x0, local_pages = {
next = 0xefef6910, prev = 0xefef6910}, allocation_order = 0, nr_local_pages = 0,
binfmt = 0x0, exit_code = 0, exit_signal = 0, pdeath_signal = 0, personality = 0,
did_exec = 0, pid = 6, pgrp = 1, tty_old_pgrp = 0, session = 1, tgid = 0, leader = 0,
real_parent = 0xefef4700, parent = 0xefef4700, children = {next = 0xefef6958,
prev = 0xefef6958}, sibling = {next = 0xefef4798, prev = 0xefef62a0},
thread_group = {next = 0xefef62a8, prev = 0xefef47a8}, pidhash_next = 0x0,
pidhash_pprev = 0xc03e4d98, wait_chldexit = {lock = {lock = 1, magic = 3735899821},
task_list = {next = 0xefef6980, prev = 0xefef6980}}, vfork_done = 0x0,
rt_priority = 0, it_real_value = 0, it_prof_value = 0, it_virt_value = 0,
it_real_incr = 0, it_prof_incr = 0, it_virt_incr = 0, real_timer = {list = {
next = 0x0, prev = 0x0}, expires = 0, data = 4025444544,
function = 0xc0123548 <it_real_fn>}, times = {tms_utime = 0, tms_stime = 0,
tms_cutime = 0, tms_cstime = 0}, start_time = 116, per_cpu_utime = {
0 <repeats 32 times>}, per_cpu_stime = {0 <repeats 32 times>}, min_flt = 0,
maj_flt = 0, nswap = 0, cmin_flt = 0, cmaj_flt = 0, cnswap = 0, swappable = -1,
uid = 0, euid = 0, suid = 0, fsuid = 0, gid = 0, egid = 0, sgid = 0, fsgid = 0,
ngroups = 0, groups = {0 <repeats 32 times>}, cap_effective = 4294967039,
cap_inheritable = 0, cap_permitted = 4294967295, keep_capabilities = 0,
user = 0xc0363a8c, rlim = {{rlim_cur = 4294967295, rlim_max = 4294967295}, {
...

It's in state TASK_RUNNING.

It's 100% reproducible. Happens with gcc-2.91.66 and gcc-3.0.3.
Can diagnose further if needed.

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