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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 "linux/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 */
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