hi,

the migration process may be hang when you start another VM while the migration is under way.

Eg.

Start a VM:

# xm create OVM_EL5U3_X86_PVHVM_4GB/vm.cfg

Wait for the VM bootup, then start another VM while this VM is migration:

# xm migrate -l OVM_EL5U3_X86_PVHVM_4GB localhost &
# xm create OVM_EL5U1_X86_HVM_4GB/vm.cfg

The migration will hang on:

# xm list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0                                     0   543     2     r-----    110.1
OVM_EL5U1_X86_HVM_4GB                        4   256     1     -b----     39.6
OVM_EL5U3_X86_PVHVM_4GB                      5   512     1     --p---      0.0

The migration will only finish after shutting down VM OVM_EL5U1_X86_HVM_4GB.

This is because we are using a threaded model in xend, and the migration thread is running
in the same context, and we are using pipes/sockets in migration.

So the write side of the pipe/socket created while migration will inherited to children, thus
the reading side of the pipe/socket will hang if there is a child forked and the child never end up
(like qemu-dm).

So we should close all inherited open file descriptors in every forked process.

Ian: please review the patch against qemu-dm forking.

Signed-off-by: Zhigang Wang <zhigang.x.wang@oracle.com>
Reviewed-by: Xiaowei Hu <xiaowei.hu@oracle.com>

thanks,

zhigang