Contents of /alx-src/tags/kernel26-2.6.12-alx-r9/Documentation/locks.txt
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Wed Mar 4 11:03:09 2009 UTC (15 years, 6 months ago) by niro
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Tag kernel26-2.6.12-alx-r9
1 | File Locking Release Notes |
2 | |
3 | Andy Walker <andy@lysaker.kvaerner.no> |
4 | |
5 | 12 May 1997 |
6 | |
7 | |
8 | 1. What's New? |
9 | -------------- |
10 | |
11 | 1.1 Broken Flock Emulation |
12 | -------------------------- |
13 | |
14 | The old flock(2) emulation in the kernel was swapped for proper BSD |
15 | compatible flock(2) support in the 1.3.x series of kernels. With the |
16 | release of the 2.1.x kernel series, support for the old emulation has |
17 | been totally removed, so that we don't need to carry this baggage |
18 | forever. |
19 | |
20 | This should not cause problems for anybody, since everybody using a |
21 | 2.1.x kernel should have updated their C library to a suitable version |
22 | anyway (see the file "Documentation/Changes".) |
23 | |
24 | 1.2 Allow Mixed Locks Again |
25 | --------------------------- |
26 | |
27 | 1.2.1 Typical Problems - Sendmail |
28 | --------------------------------- |
29 | Because sendmail was unable to use the old flock() emulation, many sendmail |
30 | installations use fcntl() instead of flock(). This is true of Slackware 3.0 |
31 | for example. This gave rise to some other subtle problems if sendmail was |
32 | configured to rebuild the alias file. Sendmail tried to lock the aliases.dir |
33 | file with fcntl() at the same time as the GDBM routines tried to lock this |
34 | file with flock(). With pre 1.3.96 kernels this could result in deadlocks that, |
35 | over time, or under a very heavy mail load, would eventually cause the kernel |
36 | to lock solid with deadlocked processes. |
37 | |
38 | |
39 | 1.2.2 The Solution |
40 | ------------------ |
41 | The solution I have chosen, after much experimentation and discussion, |
42 | is to make flock() and fcntl() locks oblivious to each other. Both can |
43 | exists, and neither will have any effect on the other. |
44 | |
45 | I wanted the two lock styles to be cooperative, but there were so many |
46 | race and deadlock conditions that the current solution was the only |
47 | practical one. It puts us in the same position as, for example, SunOS |
48 | 4.1.x and several other commercial Unices. The only OS's that support |
49 | cooperative flock()/fcntl() are those that emulate flock() using |
50 | fcntl(), with all the problems that implies. |
51 | |
52 | |
53 | 1.3 Mandatory Locking As A Mount Option |
54 | --------------------------------------- |
55 | |
56 | Mandatory locking, as described in 'Documentation/mandatory.txt' was prior |
57 | to this release a general configuration option that was valid for all |
58 | mounted filesystems. This had a number of inherent dangers, not the least |
59 | of which was the ability to freeze an NFS server by asking it to read a |
60 | file for which a mandatory lock existed. |
61 | |
62 | From this release of the kernel, mandatory locking can be turned on and off |
63 | on a per-filesystem basis, using the mount options 'mand' and 'nomand'. |
64 | The default is to disallow mandatory locking. The intention is that |
65 | mandatory locking only be enabled on a local filesystem as the specific need |
66 | arises. |
67 | |
68 | Until an updated version of mount(8) becomes available you may have to apply |
69 | this patch to the mount sources (based on the version distributed with Rick |
70 | Faith's util-linux-2.5 package): |
71 | |
72 | *** mount.c.orig Sat Jun 8 09:14:31 1996 |
73 | --- mount.c Sat Jun 8 09:13:02 1996 |
74 | *************** |
75 | *** 100,105 **** |
76 | --- 100,107 ---- |
77 | { "noauto", 0, MS_NOAUTO }, /* Can only be mounted explicitly */ |
78 | { "user", 0, MS_USER }, /* Allow ordinary user to mount */ |
79 | { "nouser", 1, MS_USER }, /* Forbid ordinary user to mount */ |
80 | + { "mand", 0, MS_MANDLOCK }, /* Allow mandatory locks on this FS */ |
81 | + { "nomand", 1, MS_MANDLOCK }, /* Forbid mandatory locks on this FS */ |
82 | /* add new options here */ |
83 | #ifdef MS_NOSUB |
84 | { "sub", 1, MS_NOSUB }, /* allow submounts */ |