Adds a lock to protect the sm pending_sends list from concurrent access
Fixes bug where btl_sm_process_pending_sends would return an item to
the free list and then continue to use it for a little while
cmr:v1.6
This commit was SVN r25878.
The following Trac tickets were found above:
Ticket 2998 --> https://svn.open-mpi.org/trac/ompi/ticket/2998