Makefile / racy-git.txt: clarify USE_NSEC prerequisites

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/racy-git.txt

@@ -41,13 +41,17 @@ With a `USE_STDEV` compile-time option, `st_dev` is also
 compared, but this is not enabled by default because this member
 is not stable on network filesystems.  With `USE_NSEC`
 compile-time option, `st_mtim.tv_nsec` and `st_ctim.tv_nsec`
-members are also compared, but this is not enabled by default
+members are also compared. On Linux, this is not enabled by default
 because in-core timestamps can have finer granularity than
 on-disk timestamps, resulting in meaningless changes when an
 inode is evicted from the inode cache.  See commit 8ce13b0
 of git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
 ([PATCH] Sync in core time granularity with filesystems,
-2005-01-04).
+2005-01-04). This patch is included in kernel 2.6.11 and newer, but
+only fixes the issue for file systems with exactly 1 ns or 1 s
+resolution. Other file systems are still broken in current Linux
+kernels (e.g. CEPH, CIFS, NTFS, UDF), see
+https://lkml.org/lkml/2015/6/9/714
 
 Racy Git
 --------

Makefile

@@ -217,10 +217,11 @@ all::
 # as the compiler can crash (http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49299)
 #
 # Define USE_NSEC below if you want git to care about sub-second file mtimes
-# and ctimes. Note that you need recent glibc (at least 2.2.4) for this, and
-# it will BREAK YOUR LOCAL DIFFS! show-diff and anything using it will likely
-# randomly break unless your underlying filesystem supports those sub-second
-# times (my ext3 doesn't).
+# and ctimes. Note that you need recent glibc (at least 2.2.4) for this. On
+# Linux, kernel 2.6.11 or newer is required for reliable sub-second file times
+# on file systems with exactly 1 ns or 1 s resolution. If you intend to use Git
+# on other file systems (e.g. CEPH, CIFS, NTFS, UDF), don't enable USE_NSEC. See
+# Documentation/technical/racy-git.txt for details.
 #
 # Define USE_ST_TIMESPEC if your "struct stat" uses "st_ctimespec" instead of
 # "st_ctim"