About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / filesystems / locks.txt




Custom Search

Based on kernel version 3.2. Page generated on 2012-01-05 23:28 EST.

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
57	'Documentation/filesystems/mandatory-locking.txt' was prior to this release a
58	general configuration option that was valid for all mounted filesystems.  This
59	had a number of inherent dangers, not the least of which was the ability to
60	freeze an NFS server by asking it to read a file for which a mandatory lock
61	existed.
62	
63	From this release of the kernel, mandatory locking can be turned on and off
64	on a per-filesystem basis, using the mount options 'mand' and 'nomand'.
65	The default is to disallow mandatory locking. The intention is that
66	mandatory locking only be enabled on a local filesystem as the specific need
67	arises.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog
Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.