[openib-general] CMA issue: bind selects the same port after close

[Sean Hefty]

Michael S. Tsirkin wrote:
> We have encountered an issue in CMA: if
> I bind to port 0, destroy the id, then bind to port 0 again
> I often get back the same port from both binds.
> 
> TCP behaves differently - it seems to assign  new port numbers
> each time.
> This is an issue for some socket programs that assume that
> the same port number won't be reused to a remote side that
> connects to the same port after I have  closed by socket will get
> connection refused message.
> I also see applications looking for a port number that matches
> some rule by repeating the create/bind/close cycle.
> With CMA they always get back the same port number it seems.
> 
> Is this something that can be fixed in CMA?

I think we can fix this without a huge impact.  Is there anything that states 
the way bind is supposed to behave wrt this?  Is there some delay between 
releasing a port and it being re-used that needs to be taken into account?

The basic problem in the CMA is in cma_alloc_port().  If the port number (passed 
in as snum) is 0, the first available port starting at 
sysctl_local_port_range[0] is used.  We could instead start our search by 
adding an increasing counter or a random value to the lower-end of the port 
range.  Then expand the code to handle searching below our starting value if we 
failed to find one above it.

Are the port numbers assigned by TCP sequential or more random?

- Sean