linux/Documentation/filesystems/locks.rst
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   1.. SPDX-License-Identifier: GPL-2.0
   2
   3==========================
   4File Locking Release Notes
   5==========================
   6
   7                Andy Walker <andy@lysaker.kvaerner.no>
   8
   9                            12 May 1997
  10
  11
  121. What's New?
  13==============
  14
  151.1 Broken Flock Emulation
  16--------------------------
  17
  18The old flock(2) emulation in the kernel was swapped for proper BSD
  19compatible flock(2) support in the 1.3.x series of kernels. With the
  20release of the 2.1.x kernel series, support for the old emulation has
  21been totally removed, so that we don't need to carry this baggage
  22forever.
  23
  24This should not cause problems for anybody, since everybody using a
  252.1.x kernel should have updated their C library to a suitable version
  26anyway (see the file "Documentation/process/changes.rst".)
  27
  281.2 Allow Mixed Locks Again
  29---------------------------
  30
  311.2.1 Typical Problems - Sendmail
  32^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  33Because sendmail was unable to use the old flock() emulation, many sendmail
  34installations use fcntl() instead of flock(). This is true of Slackware 3.0
  35for example. This gave rise to some other subtle problems if sendmail was
  36configured to rebuild the alias file. Sendmail tried to lock the aliases.dir
  37file with fcntl() at the same time as the GDBM routines tried to lock this
  38file with flock(). With pre 1.3.96 kernels this could result in deadlocks that,
  39over time, or under a very heavy mail load, would eventually cause the kernel
  40to lock solid with deadlocked processes.
  41
  42
  431.2.2 The Solution
  44^^^^^^^^^^^^^^^^^^
  45The solution I have chosen, after much experimentation and discussion,
  46is to make flock() and fcntl() locks oblivious to each other. Both can
  47exists, and neither will have any effect on the other.
  48
  49I wanted the two lock styles to be cooperative, but there were so many
  50race and deadlock conditions that the current solution was the only
  51practical one. It puts us in the same position as, for example, SunOS
  524.1.x and several other commercial Unices. The only OS's that support
  53cooperative flock()/fcntl() are those that emulate flock() using
  54fcntl(), with all the problems that implies.
  55
  56
  571.3 Mandatory Locking As A Mount Option
  58---------------------------------------
  59
  60Mandatory locking was prior to this release a general configuration option
  61that was valid for all mounted filesystems.  This had a number of inherent
  62dangers, not the least of which was the ability to freeze an NFS server by
  63asking it to read a file for which a mandatory lock existed.
  64
  65Such option was dropped in Kernel v5.14.
  66