File Locking Release Notes

		Andy Walker <andy@lysaker.kvaerner.no>

			    15 May 1996


What's New?
-----------

Flock Emulation Warnings
------------------------
Many people will have noticed the ugly messages that the file locking
code started generating with the release of kernel version 1.3.95. The
messages look something like this:

    fcntl_setlk() called by process XX with broken flock() emulation

This is a warning for people using older C libraries that those libraries
are still calling the pre 1.3.x flock() emulation routines, instead of
the real flock() system call. The old routines are quite badly broken,
especially with respect to parent-child lock sharing, and can give bad
results if, for example, sendmail attempts to use them.

Fixed versions of the C libraries have been on public release for many
months. The latest versions are 5.2.18 or 5.3.12 for ELF, and I believe
somebody made a 4.7.6 release for people using a.out systems.

In 1.3.96 Linus decided to be lenient on the stragglers and changed the
warning message so that the kernel will only complain five times and
then shut up. That should make life more bearable even for people who,
for some reason, don't want to upgrade.

Sendmail Problems
-----------------
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.

Disallow Mixed Locks
--------------------
I have chosen the rather cruel solution of disallowing mixed locking styles
on a given file at a given time. Attempts to lock a file with flock() when
fcntl() locks exist, or vice versa, return with an error status of EBUSY.
This seemed to be the only way to avoid all possible deadlock conditions,
as flock() locks do not strictly have one owner process and so can't be
checked for deadlocking in the usual manner.

The process that created a lock with flock() might have forked multiple
children and exited. Previously the parent process would have been marked
as the owner of the lock, but deadlocks could just have easily occurred in
one or more of the children, which we would not have been able to identify
and avoid.

Some programs may break (again, groan). In particular the aforementioned
sendmail may have problems running in 'newaliases' mode. It will no longer
deadlock though. Recompile sendmail to use flock() and your troubles will
be over.