I boot the system up with one disk in a two disk mirror set.  When I add 
the second disk with

mdadm /dev/md0 -a /dev/sdb1

md0_resync gets a kernel crash with 2.6.11-rc5-mm1.  This also occurs 
with 2.6.11-rc4-mm1, but not with 2.6.11-rc5.

I have attached the config file for 2.6.11-rc5.  Here is the crash dump:

md: bind<sdb1>
RAID1 conf printout:
 --- wd:1 rd:2
 disk 0, wo:0, o:1, dev:sda1
 disk 1, wo:1, o:1, dev:sdb1
....<6>md: syncing RAID array md0
md: minimum _guaranteed_ reconstruction speed: 1000 KB/sec/disc.
md: using maximum available idle IO bandwith (but not more than 200000 
KB/sec) f
or reconstruction.
md: using 128k window, over a total of 9767424 blocks.
------------[ cut here ]------------
kernel BUG at drivers/md/raid1.c:1250!
invalid operand: 0000 [#1]
PREEMPT SMP
Modules linked in: autofs4 ipv6 floppy evdev psmouse pcspkr rtc i2c_i801 
i2c_cor
e hw_random uhci_hcd shpchp pci_hotplug 3c59x mii piix ehci_hcd usbcore 
e1000 id
e_generic ide_cd cdrom ide_core xfs reiserfs ext3 jbd 3w_9xxx sd_mod 
ata_piix li
bata scsi_mod raid1 md_mod unix
CPU:    0
EIP:    0060:[<f882f39c>]    Not tainted VLI
EFLAGS: 00010297   (2.6.11-rc5-mm1)
EIP is at sync_request+0x33c/0x750 [raid1]
eax: f7a41480   ebx: 00000000   ecx: 0000000c   edx: 00000000
esi: 00001000   edi: 00000000   ebp: f7de4680   esp: f6105df8
ds: 007b   es: 007b   ss: 0068
Process md0_resync (pid: 4276, threadinfo=f6104000 task=f7227020)
Stack: f6c4b840 00000010 00000000 f6105e78 00000000 00000018 00000286 
00000000
       00001000 00000000 00000000 00000000 00000000 00000000 00000000 
00000000
       00000001 00000000 00000000 012a1400 00000000 f70416a0 f7a41480 
00000000
Call Trace:
 [<f88493cd>] md_do_sync+0x4cd/0x9b0 [md_mod]
 [<c0113808>] recalc_task_prio+0x88/0x150
 [<c0309173>] schedule+0x3e3/0xc40
 [<f8847d48>] md_thread+0x178/0x180 [md_mod]
 [<c01319f0>] autoremove_wake_function+0x0/0x60
 [<c01027f2>] ret_from_fork+0x6/0x14
 [<c01319f0>] autoremove_wake_function+0x0/0x60
 [<f8847bd0>] md_thread+0x0/0x180 [md_mod]
 [<c01009f5>] kernel_thread_helper+0x5/0x10
Code: 09 89 4c 24 20 0f 84 44 01 00 00 8b 44 24 58 8b 58 54 85 db 0f 85 
85 01 00
 00 8b 5c 24 64 85 db 0f 84 48 01 00 00 83 fb 07 77 08 <0f> 0b e2 04 91 
03 83 f8
 89 d8 c1 e0 09 39 44 24 20 0f 4e 44 24