[CIFS] add mount option to send mandatory rather than advisory locks

Some applications/subsystems require mandatory byte range locks
(as is used for Windows/DOS/OS2 etc). Sending advisory (posix style)
byte range lock requests (instead of mandatory byte range locks) can
lead to problems for these applications (which expect that other
clients be prevented from writing to portions of the file which
they have locked and are updating).  This mount option allows
mounting cifs with the new mount option "forcemand" (or
"forcemandatorylock") in order to have the cifs client use mandatory
byte range locks (ie SMB/CIFS/Windows/NTFS style locks) rather than
posix byte range lock requests, even if the server would support
posix byte range lock requests.  This has no effect if the server
does not support the CIFS Unix Extensions (since posix style locks
require support for the CIFS Unix Extensions), but for mounts
to Samba servers this can be helpful for Wine and applications
that require mandatory byte range locks.

Acked-by: Jeff Layton <jlayton@redhat.com>
CC: Alexander Bokovoy <ab@samba.org>
Signed-off-by: Steve French <sfrench@us.ibm.com>

1 files changed, 11 insertions, 1 deletions

diff --git a/fs/cifs/README b/fs/cifs/README
index a439dc1739b3..da4515e3be20 100644
--- a/fs/cifs/README
+++ b/fs/cifs/README
@@ -463,9 +463,19 @@ A partial list of the supported mount options follows:
                 with cifs style mandatory byte range locks (and most
                 cifs servers do not yet support requesting advisory
                 byte range locks).
+ forcemandatorylock Even if the server supports posix (advisory) byte range
+                locking, send only mandatory lock requests.  For some
+                (presumably rare) applications, originally coded for
+                DOS/Windows, which require Windows style mandatory byte range
+                locking, they may be able to take advantage of this option,
+                forcing the cifs client to only send mandatory locks
+                even if the cifs server would support posix advisory locks.
+                "forcemand" is accepted as a shorter form of this mount
+                option.
  nodfs          Disable DFS (global name space support) even if the
                 server claims to support it.  This can help work around
-                a problem with parsing of DFS paths with Samba 3.0.24 server.
+                a problem with parsing of DFS paths with Samba server
+                versions 3.0.24 and 3.0.25.
  remount        remount the share (often used to change from ro to rw mounts
                 or vice versa)
  cifsacl        Report mode bits (e.g. on stat) based on the Windows ACL for