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+                      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/Changes".)
+
+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/mandatory.txt' 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.
+
+Until an updated version of mount(8) becomes available you may have to apply
+this patch to the mount sources (based on the version distributed with Rick
+Faith's util-linux-2.5 package):
+
+*** mount.c.orig        Sat Jun  8 09:14:31 1996
+--- mount.c     Sat Jun  8 09:13:02 1996
+***************
+*** 100,105 ****
+--- 100,107 ----
+    { "noauto", 0, MS_NOAUTO    },      /* Can  only be mounted explicitly */
+    { "user",   0, MS_USER      },      /* Allow ordinary user to mount */
+    { "nouser", 1, MS_USER      },      /* Forbid ordinary user to mount */
++   { "mand",   0, MS_MANDLOCK  },      /* Allow mandatory locks on this FS */
++   { "nomand", 1, MS_MANDLOCK  },      /* Forbid mandatory locks on this FS */
+    /* add new options here */
+  #ifdef MS_NOSUB
+    { "sub",    1, MS_NOSUB     },      /* allow submounts */