vbucketmigrator locks when under load

With build beta1-22:
Had a cluster of 2 nodes, added 2 more. Started mc-hammer, hit rebalance, and vbucketmigrator locked up. strace showed epoll_wait(

(gdb) thread apply all bt

Thread 2 (Thread -1208435824 (LWP 16464)):
#0 0x001a3402 in __kernel_vsyscall ()
#1 0x00834a6b in read () from /lib/libc.so.6
#2 0x007d5a28 in _IO_file_read_internal () from /lib/libc.so.6
#3 0x007d6d80 in _IO_new_file_underflow () from /lib/libc.so.6
#4 0x007d747b in _IO_default_uflow_internal () from /lib/libc.so.6
#5 0x007d87ed in __uflow () from /lib/libc.so.6
#6 0x007d25cc in getc () from /lib/libc.so.6
#7 0x080505c2 in std::basic_filebuf<char, std::char_traits<char> >::showmanyc
()
#8 0x008ec46b in start_thread () from /lib/libpthread.so.0
#9 0x00843dbe in clone () from /lib/libc.so.6

Thread 1 (Thread -1208432944 (LWP 16463)):
#0 0x001a3402 in __kernel_vsyscall ()
#1 0x00844436 in epoll_wait () from /lib/libc.so.6
#2 0x080579a9 in geteuid@plt ()
#3 0x00000003 in ?? ()
#4 0x08dc44c0 in ?? ()
#5 0x00000020 in ?? ()
#6 0xffffffff in ?? ()
#7 0x08dc4818 in ?? ()
--Type <return> to continue, or q <return> to quit--
#8 0x00000004 in ?? ()
#9 0x00000001 in ?? ()
#10 0x00000000 in ?? ()
#0 0x001a3402 in __kernel_vsyscall ()