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author | 2020-04-27 23:17:07 +0200 | |
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committer | 2020-05-05 09:22:22 -0600 | |
commit | 63526525dd7a9c85786ff9f9592b6f4281ad6171 (patch) | |
tree | 5a537f8414f0863aadd2ab574d372008d5096820 /Documentation/filesystems/locks.rst | |
parent | docs: filesystems: convert fuse-io.txt to ReST (diff) | |
download | linux-63526525dd7a9c85786ff9f9592b6f4281ad6171.tar.gz linux-63526525dd7a9c85786ff9f9592b6f4281ad6171.tar.bz2 linux-63526525dd7a9c85786ff9f9592b6f4281ad6171.zip |
docs: filesystems: convert locks.txt to ReST
- Add a SPDX header;
- Adjust document and section titles;
- Add it to filesystems/index.rst.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/467b3f20e63d2640d22599b99229699b5fb79251.1588021877.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Diffstat (limited to 'Documentation/filesystems/locks.rst')
-rw-r--r-- | Documentation/filesystems/locks.rst | 72 |
1 files changed, 72 insertions, 0 deletions
diff --git a/Documentation/filesystems/locks.rst b/Documentation/filesystems/locks.rst new file mode 100644 index 000000000000..10f67fb9ce07 --- /dev/null +++ b/Documentation/filesystems/locks.rst @@ -0,0 +1,72 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========================== +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/process/changes.rst".) + +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-locking.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. + |