ia64/linux-2.6.18-xen.hg

view Documentation/mandatory.txt @ 854:950b9eb27661

usbback: fix urb interval value for interrupt urbs.

Signed-off-by: Noboru Iwamatsu <n_iwamatsu@jp.fujitsu.com>
author Keir Fraser <keir.fraser@citrix.com>
date Mon Apr 06 13:51:20 2009 +0100 (2009-04-06)
parents 831230e53067
children
line source
1 Mandatory File Locking For The Linux Operating System
3 Andy Walker <andy@lysaker.kvaerner.no>
5 15 April 1996
8 1. What is mandatory locking?
9 ------------------------------
11 Mandatory locking is kernel enforced file locking, as opposed to the more usual
12 cooperative file locking used to guarantee sequential access to files among
13 processes. File locks are applied using the flock() and fcntl() system calls
14 (and the lockf() library routine which is a wrapper around fcntl().) It is
15 normally a process' responsibility to check for locks on a file it wishes to
16 update, before applying its own lock, updating the file and unlocking it again.
17 The most commonly used example of this (and in the case of sendmail, the most
18 troublesome) is access to a user's mailbox. The mail user agent and the mail
19 transfer agent must guard against updating the mailbox at the same time, and
20 prevent reading the mailbox while it is being updated.
22 In a perfect world all processes would use and honour a cooperative, or
23 "advisory" locking scheme. However, the world isn't perfect, and there's
24 a lot of poorly written code out there.
26 In trying to address this problem, the designers of System V UNIX came up
27 with a "mandatory" locking scheme, whereby the operating system kernel would
28 block attempts by a process to write to a file that another process holds a
29 "read" -or- "shared" lock on, and block attempts to both read and write to a
30 file that a process holds a "write " -or- "exclusive" lock on.
32 The System V mandatory locking scheme was intended to have as little impact as
33 possible on existing user code. The scheme is based on marking individual files
34 as candidates for mandatory locking, and using the existing fcntl()/lockf()
35 interface for applying locks just as if they were normal, advisory locks.
37 Note 1: In saying "file" in the paragraphs above I am actually not telling
38 the whole truth. System V locking is based on fcntl(). The granularity of
39 fcntl() is such that it allows the locking of byte ranges in files, in addition
40 to entire files, so the mandatory locking rules also have byte level
41 granularity.
43 Note 2: POSIX.1 does not specify any scheme for mandatory locking, despite
44 borrowing the fcntl() locking scheme from System V. The mandatory locking
45 scheme is defined by the System V Interface Definition (SVID) Version 3.
47 2. Marking a file for mandatory locking
48 ---------------------------------------
50 A file is marked as a candidate for mandatory locking by setting the group-id
51 bit in its file mode but removing the group-execute bit. This is an otherwise
52 meaningless combination, and was chosen by the System V implementors so as not
53 to break existing user programs.
55 Note that the group-id bit is usually automatically cleared by the kernel when
56 a setgid file is written to. This is a security measure. The kernel has been
57 modified to recognize the special case of a mandatory lock candidate and to
58 refrain from clearing this bit. Similarly the kernel has been modified not
59 to run mandatory lock candidates with setgid privileges.
61 3. Available implementations
62 ----------------------------
64 I have considered the implementations of mandatory locking available with
65 SunOS 4.1.x, Solaris 2.x and HP-UX 9.x.
67 Generally I have tried to make the most sense out of the behaviour exhibited
68 by these three reference systems. There are many anomalies.
70 All the reference systems reject all calls to open() for a file on which
71 another process has outstanding mandatory locks. This is in direct
72 contravention of SVID 3, which states that only calls to open() with the
73 O_TRUNC flag set should be rejected. The Linux implementation follows the SVID
74 definition, which is the "Right Thing", since only calls with O_TRUNC can
75 modify the contents of the file.
77 HP-UX even disallows open() with O_TRUNC for a file with advisory locks, not
78 just mandatory locks. That would appear to contravene POSIX.1.
80 mmap() is another interesting case. All the operating systems mentioned
81 prevent mandatory locks from being applied to an mmap()'ed file, but HP-UX
82 also disallows advisory locks for such a file. SVID actually specifies the
83 paranoid HP-UX behaviour.
85 In my opinion only MAP_SHARED mappings should be immune from locking, and then
86 only from mandatory locks - that is what is currently implemented.
88 SunOS is so hopeless that it doesn't even honour the O_NONBLOCK flag for
89 mandatory locks, so reads and writes to locked files always block when they
90 should return EAGAIN.
92 I'm afraid that this is such an esoteric area that the semantics described
93 below are just as valid as any others, so long as the main points seem to
94 agree.
96 4. Semantics
97 ------------
99 1. Mandatory locks can only be applied via the fcntl()/lockf() locking
100 interface - in other words the System V/POSIX interface. BSD style
101 locks using flock() never result in a mandatory lock.
103 2. If a process has locked a region of a file with a mandatory read lock, then
104 other processes are permitted to read from that region. If any of these
105 processes attempts to write to the region it will block until the lock is
106 released, unless the process has opened the file with the O_NONBLOCK
107 flag in which case the system call will return immediately with the error
108 status EAGAIN.
110 3. If a process has locked a region of a file with a mandatory write lock, all
111 attempts to read or write to that region block until the lock is released,
112 unless a process has opened the file with the O_NONBLOCK flag in which case
113 the system call will return immediately with the error status EAGAIN.
115 4. Calls to open() with O_TRUNC, or to creat(), on a existing file that has
116 any mandatory locks owned by other processes will be rejected with the
117 error status EAGAIN.
119 5. Attempts to apply a mandatory lock to a file that is memory mapped and
120 shared (via mmap() with MAP_SHARED) will be rejected with the error status
121 EAGAIN.
123 6. Attempts to create a shared memory map of a file (via mmap() with MAP_SHARED)
124 that has any mandatory locks in effect will be rejected with the error status
125 EAGAIN.
127 5. Which system calls are affected?
128 -----------------------------------
130 Those which modify a file's contents, not just the inode. That gives read(),
131 write(), readv(), writev(), open(), creat(), mmap(), truncate() and
132 ftruncate(). truncate() and ftruncate() are considered to be "write" actions
133 for the purposes of mandatory locking.
135 The affected region is usually defined as stretching from the current position
136 for the total number of bytes read or written. For the truncate calls it is
137 defined as the bytes of a file removed or added (we must also consider bytes
138 added, as a lock can specify just "the whole file", rather than a specific
139 range of bytes.)
141 Note 3: I may have overlooked some system calls that need mandatory lock
142 checking in my eagerness to get this code out the door. Please let me know, or
143 better still fix the system calls yourself and submit a patch to me or Linus.
145 6. Warning!
146 -----------
148 Not even root can override a mandatory lock, so runaway processes can wreak
149 havoc if they lock crucial files. The way around it is to change the file
150 permissions (remove the setgid bit) before trying to read or write to it.
151 Of course, that might be a bit tricky if the system is hung :-(