File Locking Release Notes Andy Walker 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.