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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/filesystems/mandatory-locking.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.
-