| .. 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. |