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                      File Locking Release Notes

                Andy Walker 

                            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 serveral 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

Faiths 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 */