diff options
Diffstat (limited to 'Documentation/filesystems')
-rw-r--r-- | Documentation/filesystems/00-INDEX | 2 | ||||
-rw-r--r-- | Documentation/filesystems/locks.txt | 67 |
2 files changed, 69 insertions, 0 deletions
diff --git a/Documentation/filesystems/00-INDEX b/Documentation/filesystems/00-INDEX index e801076812a4..599593a17067 100644 --- a/Documentation/filesystems/00-INDEX +++ b/Documentation/filesystems/00-INDEX | |||
@@ -52,6 +52,8 @@ isofs.txt | |||
52 | - info and mount options for the ISO 9660 (CDROM) filesystem. | 52 | - info and mount options for the ISO 9660 (CDROM) filesystem. |
53 | jfs.txt | 53 | jfs.txt |
54 | - info and mount options for the JFS filesystem. | 54 | - info and mount options for the JFS filesystem. |
55 | locks.txt | ||
56 | - info on file locking implementations, flock() vs. fcntl(), etc. | ||
55 | mandatory-locking.txt | 57 | mandatory-locking.txt |
56 | - info on the Linux implementation of Sys V mandatory file locking. | 58 | - info on the Linux implementation of Sys V mandatory file locking. |
57 | ncpfs.txt | 59 | ncpfs.txt |
diff --git a/Documentation/filesystems/locks.txt b/Documentation/filesystems/locks.txt new file mode 100644 index 000000000000..fab857accbd6 --- /dev/null +++ b/Documentation/filesystems/locks.txt | |||
@@ -0,0 +1,67 @@ | |||
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/filesystems/mandatory.txt' | ||
57 | was prior to this release a general configuration option that was valid for | ||
58 | all mounted filesystems. This had a number of inherent dangers, not the | ||
59 | least of which was the ability to freeze an NFS server by asking it to read | ||
60 | a 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 | |||