1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 ========================== 3 ==========================
4 File Locking Release Notes 4 File Locking Release Notes
5 ========================== 5 ==========================
6 6
7 Andy Walker <andy@lysaker.kvaer 7 Andy Walker <andy@lysaker.kvaerner.no>
8 8
9 12 May 1997 9 12 May 1997
10 10
11 11
12 1. What's New? 12 1. What's New?
13 ============== 13 ==============
14 14
15 1.1 Broken Flock Emulation 15 1.1 Broken Flock Emulation
16 -------------------------- 16 --------------------------
17 17
18 The old flock(2) emulation in the kernel was s 18 The old flock(2) emulation in the kernel was swapped for proper BSD
19 compatible flock(2) support in the 1.3.x serie 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 fo 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 21 been totally removed, so that we don't need to carry this baggage
22 forever. 22 forever.
23 23
24 This should not cause problems for anybody, si 24 This should not cause problems for anybody, since everybody using a
25 2.1.x kernel should have updated their C libra 25 2.1.x kernel should have updated their C library to a suitable version
26 anyway (see the file "Documentation/process/ch 26 anyway (see the file "Documentation/process/changes.rst".)
27 27
28 1.2 Allow Mixed Locks Again 28 1.2 Allow Mixed Locks Again
29 --------------------------- 29 ---------------------------
30 30
31 1.2.1 Typical Problems - Sendmail 31 1.2.1 Typical Problems - Sendmail
32 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 32 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
33 Because sendmail was unable to use the old flo 33 Because sendmail was unable to use the old flock() emulation, many sendmail
34 installations use fcntl() instead of flock(). 34 installations use fcntl() instead of flock(). This is true of Slackware 3.0
35 for example. This gave rise to some other subt 35 for example. This gave rise to some other subtle problems if sendmail was
36 configured to rebuild the alias file. Sendmail 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 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 thi 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, wo 39 over time, or under a very heavy mail load, would eventually cause the kernel
40 to lock solid with deadlocked processes. 40 to lock solid with deadlocked processes.
41 41
42 42
43 1.2.2 The Solution 43 1.2.2 The Solution
44 ^^^^^^^^^^^^^^^^^^ 44 ^^^^^^^^^^^^^^^^^^
45 The solution I have chosen, after much experim 45 The solution I have chosen, after much experimentation and discussion,
46 is to make flock() and fcntl() locks oblivious 46 is to make flock() and fcntl() locks oblivious to each other. Both can
47 exists, and neither will have any effect on th 47 exists, and neither will have any effect on the other.
48 48
49 I wanted the two lock styles to be cooperative 49 I wanted the two lock styles to be cooperative, but there were so many
50 race and deadlock conditions that the current 50 race and deadlock conditions that the current solution was the only
51 practical one. It puts us in the same position 51 practical one. It puts us in the same position as, for example, SunOS
52 4.1.x and several other commercial Unices. The 52 4.1.x and several other commercial Unices. The only OS's that support
53 cooperative flock()/fcntl() are those that emu 53 cooperative flock()/fcntl() are those that emulate flock() using
54 fcntl(), with all the problems that implies. 54 fcntl(), with all the problems that implies.
55 55
56 56
57 1.3 Mandatory Locking As A Mount Option 57 1.3 Mandatory Locking As A Mount Option
58 --------------------------------------- 58 ---------------------------------------
59 59
60 Mandatory locking was prior to this release a !! 60 Mandatory locking, as described in
61 that was valid for all mounted filesystems. T !! 61 'Documentation/filesystems/mandatory-locking.rst' was prior to this release a
62 dangers, not the least of which was the abilit !! 62 general configuration option that was valid for all mounted filesystems. This
63 asking it to read a file for which a mandatory !! 63 had a number of inherent dangers, not the least of which was the ability to
>> 64 freeze an NFS server by asking it to read a file for which a mandatory lock
>> 65 existed.
>> 66
>> 67 From this release of the kernel, mandatory locking can be turned on and off
>> 68 on a per-filesystem basis, using the mount options 'mand' and 'nomand'.
>> 69 The default is to disallow mandatory locking. The intention is that
>> 70 mandatory locking only be enabled on a local filesystem as the specific need
>> 71 arises.
64 72
65 Such option was dropped in Kernel v5.14. <<
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