| .. SPDX-License-Identifier: GPL-2.0 |
| |
| ========================== |
| 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 "Documentation/process/changes.rst".) |
| |
| 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 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. |
| |
| Such option was dropped in Kernel v5.14. |
