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Subjectrace condition between udevd and modprobe (mtrr_add)
Hi, while booting an initrd image built off of BusyBox on a thousand
nodes, we hit a race on a couple of nodes. They hang during the boot
process with the stack traces listed below. The really simple init
script in the initrd does a 'udevd --daemon' and then modprobe of a
device. The device needs to assign an mtrr to the pci resource, and
instead the whole node hangs. Putting a 'sleep 1' in between these two
calls prevents any hangs.

mtrr_add_page and the buddy allocator code don't appear to share any
semaphores, and there isn't an obvious way in which this can hang.
Possibly the smp_call_function IPI isn't being handled by the other
cores... That's the best guess. Can anyone help sort this mess out?

Also, is there a better way to test that udevd is fully up? A 'sleep
1' is not the preferred solution here.

Thanks for your time,

-Kyle Hubert

>> ps
0xffff88061d26c720 0 1036 1 0 0x400140 - udevd
0xffff88021e05c480 0 1037 1 0 0x400100 - modprobe
0xffff88081d072440 0 1116 1036 0 0x400040 - udevd
135 active task structs found
>> bt 0xffff88021e05c480
STACK TRACE FOR TASK: 0xffff88021e05c480(modprobe)

0 <schedule?> [0x0]
1 mtrr_add_page+494 [0xffffffff80219d9e]
2 <unknown?>+<ERROR> [0xffffffffa0009a08]
>> bt 0xffff88061d25f420
STACK TRACE FOR TASK: 0xffff88061d25f420(udevd)

0 <schedule?> [0x0]
1 __alloc_pages_internal+241 [0xffffffff80292731]
2 rmqueue_bulk+89 [0xffffffff80291b19]
3 get_page_from_freelist+1430 [0xffffffff802922e6]
4 __alloc_pages_internal+241 [0xffffffff80292731]
5 alloc_pages_current+168 [0xffffffff802b0898]
6 pte_alloc_one+49 [0xffffffff80229271]
7 __pte_alloc+67 [0xffffffff8029e7d3]
8 copy_page_range+1269 [0xffffffff802a11c5]
9 alloc_pid+744 [0xffffffff80250a18]
10 copy_process+3057 [0xffffffff8023bcf1]
11 do_fork+118 [0xffffffff8023c4d6]
12 sys_clone+35 [0xffffffff80209c23]
13 ptregscall_common+103 [0xffffffff8020bda7]
>> bt 0xffff88081de869e0
STACK TRACE FOR TASK: 0xffff88081de869e0(udevd)

0 <schedule?> [0x0]
1 set_user_nice+324 [0xffffffff80234734]
2 set_one_prio+113 [0xffffffff8024ea61]
3 sys_setpriority+129 [0xffffffff8024eb41]
4 system_call_fastpath+22 [0xffffffff8020ba3b]

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