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authorJ. Bruce Fields <bfields@citi.umich.edu>2007-09-30 22:18:55 -0400
committerJ. Bruce Fields <bfields@citi.umich.edu>2007-10-09 18:32:45 -0400
commit98257af5a2ad0c5b502ebd07094d9fd8ce87acef (patch)
treec8ae7b8517ad2f2d6e1cad13b9f15d6b2ebb4e64 /Documentation/filesystems
parent9efa68ed079af97f4e9477eadef567ffe64f7afc (diff)
Documentation: move locks.txt in filesystems/
This documentation (about file locking) belongs in filesystems/. Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Diffstat (limited to 'Documentation/filesystems')
-rw-r--r--Documentation/filesystems/00-INDEX2
-rw-r--r--Documentation/filesystems/locks.txt67
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
- info and mount options for the ISO 9660 (CDROM) filesystem.
jfs.txt
- info and mount options for the JFS filesystem.
+locks.txt
+ - info on file locking implementations, flock() vs. fcntl(), etc.
mandatory-locking.txt
- info on the Linux implementation of Sys V mandatory file locking.
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 @@
+ 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/Changes".)
+
+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, as described in 'Documentation/filesystems/mandatory.txt'
+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.
+
+From this release of the kernel, mandatory locking can be turned on and off
+on a per-filesystem basis, using the mount options 'mand' and 'nomand'.
+The default is to disallow mandatory locking. The intention is that
+mandatory locking only be enabled on a local filesystem as the specific need
+arises.
+