Commit 307464a1 (ofproto-dpif-monitor: Use heap to order the mport
wakeup time.) re-heapifies the heap in monitor_run(). So the
monitor_run() should be protected by the write lock, rather than
the read lock.
This commit fixes the issue.
Signed-off-by: Alex Wang <alexw@nicira.com> Signed-off-by: Ben Pfaff <blp@nicira.com>