HP CIFS File Locking Interoperation

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Chapter 4 File Locking Implementations
File systems and pseudo file systems (like PC-NFS) implement autonomous file locking
models. These file-locking models are invoked by the particular file system, and do not
necessarily interoperate, or even know each other exist. Thus, when multiple client access
methods open the same file – even with valid locking procedures for the opening file system –
the locks can be ignored, resulting in concurrent file access and likely data corruption. To
understand how the different file locking mechanisms interoperate (or do not interoperate),
we must first understand how file locking is implemented for homogeneous access on the
following file systems:
Windows (CIFS),
UNIX (JFS, HFS, etc)
NFS,
PC-NFS.
4.1 WINDOWS (CIFS)
The Windows file system (using CIFS) utilizes three different file-locking mechanisms:
Mandatory Locking,
Byte-Range Locking,
Opportunistic Locking (Oplocks).
MANDATORY LOCKING is invoked at file open by the Windows Createfile API. Locking
parameters supplied to Createfile include access mode and share mode.
Access mode defines how an application (caller of Createfile) wants to access the file:
Read: Read access only
Write: Write access only
Read-Write: Read and Write access
Share mode defines how an application wants to limit or grant concurrent access while it
has the file open, essentially comprising the locking scenario for MANDATORY LOCKS:
Deny-None: Concurrent read and write access allowed
Deny-All: No concurrent read or write access allowed
Deny-Read: No concurrent read access allowed
Deny-Write: No concurrent write access allowed
These are Windows default file locking mechanisms. There are obscure ways to disable
mandatory locks, but they should never be disabled (which is why they are obscure).
Windows documentation uses the terms “mandatory” and “share mode”. However, it is
helpful to think of mandatory share mode locking as “OPEN MODE LOCKING”, because it is
initiated during a file open, so subsequent references will use “Mandatory Share Mode (Open
Mode) Locking”.