[libvirt] [PATCH] qemu_migrate: Fix assign the same port when migrating concurrently

[Michal Privoznik]

On 27.09.2013 11:36, Wangyufei (A) wrote:
> Yes, I get your point.
> 
>             this_port = QEMUD_MIGRATION_FIRST_PORT + port++;
>             if (port == QEMUD_MIGRATION_NUM_PORTS)
>                 port = 0;

In fact this won't work either. What if this_port is already taken by
another app? virPortAllocator ensures such situation won't happen (*).

Michal

(*) Okay, I should rather say 'virPortAllocator minimizes the chance of
race' as there is still a small window for race. The virPortAllocator is
used for allocating a VNC port, for instance. So whenever a domain with
autoport for VNC is start, a free port is searched and bind()-ed. Then,
the port number is put onto qemu cmd line. And here comes the race:
libvirtd fork()s and close all FDs that are not to be transferred to the
qemu process. After this the execve("qemu", ...) is called. So the race
window begins at the FDs closing and ends when qemu decide to bind() to
the given port. But there's no better way to do this (until qemu learns
that VNC port socket can be passed as a FD).