[openib-general] RFC fix for userspace rdma cm crashes

[Sean Hefty]

There's a problem with how rdma cm events are reported to userspace that can 
lead to application crashes.

When a new connection request arrives, a context for the connection is allocated 
in the kernel.  The connection event is then reported to userspace.  The 
userspace library retrieves the event and allocates its own context for the 
connection.  The userspace context is associated with the kernel's context when 
accepting.  This allows the kernel to give userspace context with other events.

A problem occurs if a second event for the same connection occurs before the 
user has had a chance to call accept.  The userspace context has not yet been 
set, which causes the librdmacm to crash.   (This has been seen when the app 
takes too long to call accept, resulting in the remote side timing out and 
rejecting the connection.)

I can think of a couple possible fixes for this, but wanted to get input.  I 
believe that this can be fixed in either the kernel or userspace code.  A kernel 
fix could queue events until the context has been set.  A userspace fix could 
store its contexts in a map, and lookup the correct one if it is not given.

- Sean