MFC 198174: Close a race with caching of -ve name lookups in the NFS client. Specifically, clients only trust -ve cache entries while the directory remains unchanged and discard any -ve cache entries for a directory when they notice that the modification time of a directory entry changes.  The race involves two concurrent lookups as follows: - Thread A does a lookup for file 'foo' which sends a lookup RPC to the   server.  The lookup fails and the server replies. - The 'foo' file is created (either by the same client or a different   client) updating the modification time on the parent directory of 'foo'. - Thread B does a lookup for a different file 'bar' which updates the   cached attributes of the parent directory of 'foo' to reflect the new   modification time after 'foo' was created. - Thread A finally resumes execution to parse the reply from the NFS   server.  It adds a -ve cache entry and sets the cached value of the   directory's modification time that is used for invalidating -ve cached   lookups to the new modification time set by thread B.
MFC 198174:
Close a race with caching of -ve name lookups in the NFS client.
Specifically, clients only trust -ve cache entries while the directory
remains unchanged and discard any -ve cache entries for a directory when
they notice that the modification time of a directory entry changes.  The
race involves two concurrent lookups as follows:
- Thread A does a lookup for file 'foo' which sends a lookup RPC to the
  server.  The lookup fails and the server replies.
- The 'foo' file is created (either by the same client or a different
  client) updating the modification time on the parent directory of 'foo'.
- Thread B does a lookup for a different file 'bar' which updates the
  cached attributes of the parent directory of 'foo' to reflect the new
  modification time after 'foo' was created.
- Thread A finally resumes execution to parse the reply from the NFS
  server.  It adds a -ve cache entry and sets the cached value of the
  directory's modification time that is used for invalidating -ve cached
  lookups to the new modification time set by thread B.

At this point, future lookups of 'foo' will honor the -ve cached entry
until the cached entry is pushed out of the name cache's LRU or the
modification time of the parent directory is changed again by some other
change.  The fix is to read the directory's modification time before
sending the lookup RPC and use that cached modification time when setting
the directory's cached modification time.  Also, we do not add a -ve cache
entry if another thread has added -ve cache entry that set the directory's
cached modification time to a newer value than the value we read before
sending the lookup RPC.

Approved by:	re (kib)

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase