.. SPDX-License-Identifier: GPL-2.0
  
  ==========================
  File Locking Release Notes
  ==========================

  
  		Andy Walker <andy@lysaker.kvaerner.no>
  
  			    12 May 1997
  
  
  1. What's New?

  ==============

  
  1.1 Broken Flock Emulation
  --------------------------
  
  The old flock(2) emulation in the kernel was swapped for proper BSD
  compatible flock(2) support in the 1.3.x series of kernels. With the
  release of the 2.1.x kernel series, support for the old emulation has
  been totally removed, so that we don't need to carry this baggage
  forever.
  
  This should not cause problems for anybody, since everybody using a
  2.1.x kernel should have updated their C library to a suitable version

  anyway (see the file "Documentation/process/changes.rst".)

  
  1.2 Allow Mixed Locks Again
  ---------------------------
  
  1.2.1 Typical Problems - Sendmail

  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  Because sendmail was unable to use the old flock() emulation, many sendmail
  installations use fcntl() instead of flock(). This is true of Slackware 3.0
  for example. This gave rise to some other subtle problems if sendmail was
  configured to rebuild the alias file. Sendmail tried to lock the aliases.dir
  file with fcntl() at the same time as the GDBM routines tried to lock this
  file with flock(). With pre 1.3.96 kernels this could result in deadlocks that,
  over time, or under a very heavy mail load, would eventually cause the kernel
  to lock solid with deadlocked processes.
  
  
  1.2.2 The Solution

  ^^^^^^^^^^^^^^^^^^

  The solution I have chosen, after much experimentation and discussion,
  is to make flock() and fcntl() locks oblivious to each other. Both can
  exists, and neither will have any effect on the other.
  
  I wanted the two lock styles to be cooperative, but there were so many
  race and deadlock conditions that the current solution was the only
  practical one. It puts us in the same position as, for example, SunOS
  4.1.x and several other commercial Unices. The only OS's that support
  cooperative flock()/fcntl() are those that emulate flock() using
  fcntl(), with all the problems that implies.
  
  
  1.3 Mandatory Locking As A Mount Option
  ---------------------------------------

  Mandatory locking, as described in

  'Documentation/filesystems/mandatory-locking.rst' was prior to this release a

  general configuration option that was valid for all mounted filesystems.  This
  had a number of inherent dangers, not the least of which was the ability to
  freeze an NFS server by asking it to read a file for which a mandatory lock
  existed.

  
  From this release of the kernel, mandatory locking can be turned on and off
  on a per-filesystem basis, using the mount options 'mand' and 'nomand'.
  The default is to disallow mandatory locking. The intention is that
  mandatory locking only be enabled on a local filesystem as the specific need
  arises.