1 .. SPDX-License-Identifier: GPL-2.0 2 3 ========================== 4 File Locking Release Notes 5 ========================== 6 7 Andy Walker <andy@lysaker.kvaerner.no> 8 9 12 May 1997 10 11 12 1. What's New? 13 ============== 14 15 1.1 Broken Flock Emulation 16 -------------------------- 17 18 The old flock(2) emulation in the kernel was swapped for proper BSD 19 compatible flock(2) support in the 1.3.x series of kernels. With the 20 release of the 2.1.x kernel series, support for the old emulation has 21 been totally removed, so that we don't need to carry this baggage 22 forever. 23 24 This should not cause problems for anybody, since everybody using a 25 2.1.x kernel should have updated their C library to a suitable version 26 anyway (see the file "Documentation/process/changes.rst".) 27 28 1.2 Allow Mixed Locks Again 29 --------------------------- 30 31 1.2.1 Typical Problems - Sendmail 32 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 33 Because sendmail was unable to use the old flock() emulation, many sendmail 34 installations use fcntl() instead of flock(). This is true of Slackware 3.0 35 for example. This gave rise to some other subtle problems if sendmail was 36 configured to rebuild the alias file. Sendmail tried to lock the aliases.dir 37 file with fcntl() at the same time as the GDBM routines tried to lock this 38 file with flock(). With pre 1.3.96 kernels this could result in deadlocks that, 39 over time, or under a very heavy mail load, would eventually cause the kernel 40 to lock solid with deadlocked processes. 41 42 43 1.2.2 The Solution 44 ^^^^^^^^^^^^^^^^^^ 45 The solution I have chosen, after much experimentation and discussion, 46 is to make flock() and fcntl() locks oblivious to each other. Both can 47 exists, and neither will have any effect on the other. 48 49 I wanted the two lock styles to be cooperative, but there were so many 50 race and deadlock conditions that the current solution was the only 51 practical one. It puts us in the same position as, for example, SunOS 52 4.1.x and several other commercial Unices. The only OS's that support 53 cooperative flock()/fcntl() are those that emulate flock() using 54 fcntl(), with all the problems that implies. 55 56 57 1.3 Mandatory Locking As A Mount Option 58 --------------------------------------- 59 60 Mandatory locking was prior to this release a general configuration option 61 that was valid for all mounted filesystems. This had a number of inherent 62 dangers, not the least of which was the ability to freeze an NFS server by 63 asking it to read a file for which a mandatory lock existed. 64 65 Such option was dropped in Kernel v5.14.
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