If two attach requests to the same piece of shared memory occur,
a double export or premature dmabuf fd close can occur since the export
and close on demand calls are not atomic.
Use a reference counter on shared memory dmabuf FDs that have
already been opened to avoid this problem.
Change-Id: I14a59209c0385e32582af42a57b33b1c6838a9b1
Jonathan Kim
1f63ea3476