root/Documentation/locks.txt

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                      File Locking Release Notes

                Andy Walker <andy@lysaker.kvaerner.no>

                           15 April 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.

I have chosen the rather cruel solution of returning an error when such a
deadlock would occur. I can't see any other way to handle this situation
gracefully. The other options are to maintain two entirely separate lists
for flock() and fcntl() locks, thus defeating any protection between the
two, or to free locks placed by one method when the same process later
tries to lock the same file by the other method. Neither option seems
satisfactory.

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.

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