linux/Documentation/filesystems/locks.txt
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   1                      File Locking Release Notes
   2
   3                Andy Walker <andy@lysaker.kvaerner.no>
   4
   5                            12 May 1997
   6
   7
   81. What's New?
   9--------------
  10
  111.1 Broken Flock Emulation
  12--------------------------
  13
  14The old flock(2) emulation in the kernel was swapped for proper BSD
  15compatible flock(2) support in the 1.3.x series of kernels. With the
  16release of the 2.1.x kernel series, support for the old emulation has
  17been totally removed, so that we don't need to carry this baggage
  18forever.
  19
  20This should not cause problems for anybody, since everybody using a
  212.1.x kernel should have updated their C library to a suitable version
  22anyway (see the file "Documentation/Changes".)
  23
  241.2 Allow Mixed Locks Again
  25---------------------------
  26
  271.2.1 Typical Problems - Sendmail
  28---------------------------------
  29Because sendmail was unable to use the old flock() emulation, many sendmail
  30installations use fcntl() instead of flock(). This is true of Slackware 3.0
  31for example. This gave rise to some other subtle problems if sendmail was
  32configured to rebuild the alias file. Sendmail tried to lock the aliases.dir
  33file with fcntl() at the same time as the GDBM routines tried to lock this
  34file with flock(). With pre 1.3.96 kernels this could result in deadlocks that,
  35over time, or under a very heavy mail load, would eventually cause the kernel
  36to lock solid with deadlocked processes.
  37
  38
  391.2.2 The Solution
  40------------------
  41The solution I have chosen, after much experimentation and discussion,
  42is to make flock() and fcntl() locks oblivious to each other. Both can
  43exists, and neither will have any effect on the other.
  44
  45I wanted the two lock styles to be cooperative, but there were so many
  46race and deadlock conditions that the current solution was the only
  47practical one. It puts us in the same position as, for example, SunOS
  484.1.x and several other commercial Unices. The only OS's that support
  49cooperative flock()/fcntl() are those that emulate flock() using
  50fcntl(), with all the problems that implies.
  51
  52
  531.3 Mandatory Locking As A Mount Option
  54---------------------------------------
  55
  56Mandatory locking, as described in
  57'Documentation/filesystems/mandatory-locking.txt' was prior to this release a
  58general configuration option that was valid for all mounted filesystems.  This
  59had a number of inherent dangers, not the least of which was the ability to
  60freeze an NFS server by asking it to read a file for which a mandatory lock
  61existed.
  62
  63From this release of the kernel, mandatory locking can be turned on and off
  64on a per-filesystem basis, using the mount options 'mand' and 'nomand'.
  65The default is to disallow mandatory locking. The intention is that
  66mandatory locking only be enabled on a local filesystem as the specific need
  67arises.
  68
  69
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