fix a race condition in Lwt_io
fix a race condition in Lwt_io

Ignore-this: 5c61f64ae00bfa488a03441de12de629

darcs-hash:20110718145731-c41ad-7ced2812fef6069e83864996a274e5e55db1aaf4

mfp/lwt,ocsigen/lwt,janestreet/lwt-async,mfp/lwt,rneswold/lwt,c-cube/lwt,rneswold/lwt,ocsigen/lwt,gfxmonk/lwt,rgrinberg/lwt,rgrinberg/lwt,rgrinberg/lwt,fdopen/lwt,ocsigen/lwt,fdopen/lwt,c-cube/lwt,rgrinberg/lwt,c-cube/lwt,gfxmonk/lwt,mirage/lwt,mirage/lwt,rneswold/lwt,janestreet/lwt-async