asynchronous kernel write vs shadow write timing issues

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on May 23, 2011 10:58:25_

today for asynch syscalls I don't have a perfect solution: I presently mark
as written post-syscall and don't wait for the completion event. what I'm
doing now can in addition to being early also be late: if event is
delivered prior to syscall return. the latter should be looked at. for
being early the only problem is if another thread goes and reads from the
buffer and we fail to raise an unint error.

in some cases the actual size written may not be available until the completion event.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=410_