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Editing: locking.txt
/* ======================================================================== * Copyright 1988-2006 University of Washington * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * * ======================================================================== */ UNIX Advisory File Locking Implications on c-client Mark Crispin, 28 November 1995 THIS DOCUMENT HAS BEEN UPDATED TO REFLECT THE FACT THAT LINUX SUPPORTS BOTH flock() AND fcntl() AND THAT OSF/1 HAS BEEN BROKEN SO THAT IT ONLY SUPPORTS fcntl(). -- JUNE 15, 2004 THIS DOCUMENT HAS BEEN UPDATED TO REFLECT THE CODE IN THE IMAP-4 TOOLKIT AS OF NOVEMBER 28, 1995. SOME STATEMENTS IN THIS DOCUMENT DO NOT APPLY TO EARLIER VERSIONS OF THE IMAP TOOLKIT. INTRODUCTION Advisory locking is a mechanism by which cooperating processes can signal to each other their usage of a resource and whether or not that usage is critical. It is not a mechanism to protect against processes which do not cooperate in the locking. The most basic form of locking involves a counter. This counter is -1 when the resource is available. If a process wants the lock, it executes an atomic increment-and-test-if-zero. If the value is zero, the process has the lock and can execute the critical code that needs exclusive usage of a resource. When it is finished, it sets the lock back to -1. In C terms: while (++lock) /* try to get lock */ invoke_other_threads (); /* failed, try again */ . . /* critical code here */ . lock = -1; /* release lock */ This particular form of locking appears most commonly in multi-threaded applications such as operating system kernels. It makes several presumptions: (1) it is alright to keep testing the lock (no overflow) (2) the critical resource is single-access only (3) there is shared writeable memory between the two threads (4) the threads can be trusted to release the lock when finished In applications programming on multi-user systems, most commonly the other threads are in an entirely different process, which may even be logged in as a different user. Few operating systems offer shared writeable memory between such processes. A means of communicating this is by use of a file with a mutually agreed upon name. A binary semaphore can be passed by means of the existance or non-existance of that file, provided that there is an atomic means to create a file if and only if that file does not exist. In C terms: /* try to get lock */ while ((fd = open ("lockfile",O_WRONLY|O_CREAT|O_EXCL,0666)) < 0) sleep (1); /* failed, try again */ close (fd); /* got the lock */ . . /* critical code here */ . unlink ("lockfile"); /* release lock */ This form of locking makes fewer presumptions, but it still is guilty of presumptions (2) and (4) above. Presumption (2) limits the ability to have processes sharing a resource in a non-conflicting fashion (e.g. reading from a file). Presumption (4) leads to deadlocks should the process crash while it has a resource locked. Most modern operating systems provide a resource locking system call that has none of these presumptions. In particular, a mechanism is provided for identifying shared locks as opposed to exclusive locks. A shared lock permits other processes to obtain a shared lock, but denies exclusive locks. In other words: current state want shared want exclusive ------------- ----------- -------------- unlocked YES YES locked shared YES NO locked exclusive NO NO Furthermore, the operating system automatically relinquishes all locks held by that process when it terminates. A useful operation is the ability to upgrade a shared lock to exclusive (provided there are no other shared users of the lock) and to downgrade an exclusive lock to shared. It is important that at no time is the lock ever removed; a process upgrading to exclusive must not relenquish its shared lock. Most commonly, the resources being locked are files. Shared locks are particularly important with files; multiple simultaneous processes can read from a file, but only one can safely write at a time. Some writes may be safer than others; an append to the end of the file is safer than changing existing file data. In turn, changing a file record in place is safer than rewriting the file with an entirely different structure. FILE LOCKING ON UNIX In the oldest versions of UNIX, the use of a semaphore lockfile was the only available form of locking. Advisory locking system calls were not added to UNIX until after the BSD vs. System V split. Both of these system calls deal with file resources only. Most systems only have one or the other form of locking. AIX and newer versions of OSF/1 emulate the BSD form of locking as a jacket into the System V form. Ultrix and Linux implement both forms. BSD BSD added the flock() system call. It offers capabilities to acquire shared lock, acquire exclusive lock, and unlock. Optionally, the process can request an immediate error return instead of blocking when the lock is unavailable. FLOCK() BUGS flock() advertises that it permits upgrading of shared locks to exclusive and downgrading of exclusive locks to shared, but it does so by releasing the former lock and then trying to acquire the new lock. This creates a window of vulnerability in which another process can grab the exclusive lock. Therefore, this capability is not useful, although many programmers have been deluded by incautious reading of the flock() man page to believe otherwise. This problem can be programmed around, once the programmer is aware of it. flock() always returns as if it succeeded on NFS files, when in fact it is a no-op. There is no way around this. Leaving aside these two problems, flock() works remarkably well, and has shown itself to be robust and trustworthy. SYSTEM V/POSIX System V added new functions to the fnctl() system call, and a simple interface through the lockf() subroutine. This was subsequently included in POSIX. Both offer the facility to apply the lock to a particular region of the file instead of to the entire file. lockf() only supports exclusive locks, and calls fcntl() internally; hence it won't be discussed further. Functionally, fcntl() locking is a superset of flock(); it is possible to implement a flock() emulator using fcntl(), with one minor exception: it is not possible to acquire an exclusive lock if the file is not open for write. The fcntl() locking functions are: query lock station of a file region, lock/unlock a region, and lock/unlock a region and block until have the lock. The locks may be shared or exclusive. By means of the statd and lockd daemons, fcntl() locking is available on NFS files. When statd is started at system boot, it reads its /etc/state file (which contains the number of times it has been invoked) and /etc/sm directory (which contains a list of all remote sites which are client or server locking with this site), and notifies the statd on each of these systems that it has been restarted. Each statd then notifies the local lockd of the restart of that system. lockd receives fcntl() requests for NFS files. It communicates with the lockd at the server and requests it to apply the lock, and with the statd to request it for notification when the server goes down. It blocks until all these requests are completed. There is quite a mythos about fcntl() locking. One religion holds that fcntl() locking is the best thing since sliced bread, and that programs which use flock() should be converted to fcntl() so that NFS locking will work. However, as noted above, very few systems support both calls, so such an exercise is pointless except on Ultrix and Linux. Another religion, which I adhere to, has the opposite viewpoint. FCNTL() BUGS For all of the hairy code to do individual section locking of a file, it's clear that the designers of fcntl() locking never considered some very basic locking operations. It's as if all they knew about locking they got out of some CS textbook with not investigation of real-world needs. It is not possible to acquire an exclusive lock unless the file is open for write. You could have append with shared read, and thus you could have a case in which a read-only access may need to go exclusive. This problem can be programmed around once the programmer is aware of it. If the file is opened on another file designator in the same process, the file is unlocked even if no attempt is made to do any form of locking on the second designator. This is a very bad bug. It means that an application must keep track of all the files that it has opened and locked. If there is no statd/lockd on the NFS server, fcntl() will hang forever waiting for them to appear. This is a bad bug. It means that any attempt to lock on a server that doesn't run these daemons will hang. There is no way for an application to request flock() style ``try to lock, but no-op if the mechanism ain't there''. There is a rumor to the effect that fcntl() will hang forever on local files too if there is no local statd/lockd. These daemons are running on mailer.u, although they appear not to have much CPU time. A useful experiment would be to kill them and see if imapd is affected in any way, but I decline to do so without an OK from UCS! ;-) If killing statd/lockd can be done without breaking fcntl() on local files, this would become one of the primary means of dealing with this problem. The statd and lockd daemons have quite a reputation for extreme fragility. There have been numerous reports about the locking mechanism being wedged on a systemwide or even clusterwide basis, requiring a reboot to clear. It is rumored that this wedge, once it happens, also blocks local locking. Presumably killing and restarting statd would suffice to clear the wedge, but I haven't verified this. There appears to be a limit to how many locks may be in use at a time on the system, although the documentation only mentions it in passing. On some of their systems, UCS has increased lockd's ``size of the socket buffer'', whatever that means. C-CLIENT USAGE c-client uses flock(). On System V systems, flock() is simulated by an emulator that calls fcntl(). BEZERK AND MMDF Locking in the traditional UNIX formats was largely dictated by the status quo in other applications; however, additional protection is added against inadvertantly running multiple instances of a c-client application on the same mail file. (1) c-client attempts to create a .lock file (mail file name with ``.lock'' appended) whenever it reads from, or writes to, the mail file. This is an exclusive lock, and is held only for short periods of time while c-client is actually doing the I/O. There is a 5-minute timeout for this lock, after which it is broken on the presumption that it is a stale lock. If it can not create the .lock file due to an EACCES (protection failure) error, it once silently proceeded without this lock; this was for systems which protect /usr/spool/mail from unprivileged processes creating files. Today, c-client reports an error unless it is built otherwise. The purpose of this lock is to prevent against unfavorable interactions with mail delivery. (2) c-client applies a shared flock() to the mail file whenever it reads from the mail file, and an exclusive flock() whenever it writes to the mail file. This lock is freed as soon as it finishes reading. The purpose of this lock is to prevent against unfavorable interactions with mail delivery. (3) c-client applies an exclusive flock() to a file on /tmp (whose name represents the device and inode number of the file) when it opens the mail file. This lock is maintained throughout the session, although c-client has a feature (called ``kiss of death'') which permits c-client to forcibly and irreversibly seize the lock from a cooperating c-client application that surrenders the lock on demand. The purpose of this lock is to prevent against unfavorable interactions with other instances of c-client (rewriting the mail file). Mail delivery daemons use lock (1), (2), or both. Lock (1) works over NFS; lock (2) is the only one that works on sites that protect /usr/spool/mail against unprivileged file creation. Prudent mail delivery daemons use both forms of locking, and of course so does c-client. If only lock (2) is used, then multiple processes can read from the mail file simultaneously, although in real life this doesn't really change things. The normal state of locks (1) and (2) is unlocked except for very brief periods. TENEX AND MTX The design of the locking mechanism of these formats was motivated by a design to enable multiple simultaneous read/write access. It is almost the reverse of how locking works with bezerk/mmdf. (1) c-client applies a shared flock() to the mail file when it opens the mail file. It upgrades this lock to exclusive whenever it tries to expunge the mail file. Because of the flock() bug that upgrading a lock actually releases it, it will not do so until it has acquired an exclusive lock (2) first. The purpose of this lock is to prevent against expunge taking place while some other c-client has the mail file open (and thus knows where all the messages are). (2) c-client applies a shared flock() to a file on /tmp (whose name represents the device and inode number of the file) when it parses the mail file. It applies an exclusive flock() to this file when it appends new mail to the mail file, as well as before it attempts to upgrade lock (1) to exclusive. The purpose of this lock is to prevent against data being appended while some other c-client is parsing mail in the file (to prevent reading of incomplete messages). It also protects against the lock-releasing timing race on lock (1). OBSERVATIONS In a perfect world, locking works. You are protected against unfavorable interactions with the mailer and against your own mistake by running more than one instance of your mail reader. In tenex/mtx formats, you have the additional benefit that multiple simultaneous read/write access works, with the sole restriction being that you can't expunge if there are any sharers of the mail file. If the mail file is NFS-mounted, then flock() locking is a silent no-op. This is the way BSD implements flock(), and c-client's emulation of flock() through fcntl() tests for NFS files and duplicates this functionality. There is no locking protection for tenex/mtx mail files at all, and only protection against the mailer for bezerk/mmdf mail files. This has been the accepted state of affairs on UNIX for many sad years. If you can not create .lock files, it should not affect locking, since the flock() locks suffice for all protection. This is, however, not true if the mailer does not check for flock() locking, or if the the mail file is NFS-mounted. What this means is that there is *no* locking protection at all in the case of a client using an NFS-mounted /usr/spool/mail that does not permit file creation by unprivileged programs. It is impossible, under these circumstances, for an unprivileged program to do anything about it. Worse, if EACCES errors on .lock file creation are no-op'ed , the user won't even know about it. This is arguably a site configuration error. The problem with not being able to create .lock files exists on System V as well, but the failure modes for flock() -- which is implemented via fcntl() -- are different. On System V, if the mail file is NFS-mounted and either the client or the server lacks a functioning statd/lockd pair, then the lock attempt would have hung forever if it weren't for the fact that c-client tests for NFS and no-ops the flock() emulator in this case. Systemwide or clusterwide failures of statd/lockd have been known to occur which cause all locks in all processes to hang (including local?). Without the special NFS test made by c-client, there would be no way to request BSD-style no-op behavior, nor is there any way to determine that this is happening other than the system being hung. The additional locking introduced by c-client was shown to cause much more stress on the System V locking mechanism than has traditionally been placed upon it. If it was stressed too far, all hell broke loose. Fortunately, this is now past history. TRADEOFFS c-client based applications have a reasonable chance of winning as long as you don't use NFS for remote access to mail files. That's what IMAP is for, after all. It is, however, very important to realize that you can *not* use the lock-upgrade feature by itself because it releases the lock as an interim step -- you need to have lock-upgrading guarded by another lock. If you have the misfortune of using System V, you are likely to run into problems sooner or later having to do with statd/lockd. You basically end up with one of three unsatisfactory choices: 1) Grit your teeth and live with it. 2) Try to make it work: a) avoid NFS access so as not to stress statd/lockd. b) try to understand the code in statd/lockd and hack it to be more robust. c) hunt out the system limit of locks, if there is one, and increase it. Figure on at least two locks per simultaneous imapd process and four locks per Pine process. Better yet, make the limit be 10 times the maximum number of processes. d) increase the socket buffer (-S switch to lockd) if it is offered. I don't know what this actually does, but giving lockd more resources to do its work can't hurt. Maybe. 3) Decide that it can't possibly work, and turn off the fcntl() calls in your program. 4) If nuking statd/lockd can be done without breaking local locking, then do so. This would make SVR4 have the same limitations as BSD locking, with a couple of additional bugs. 5) Check for NFS, and don't do the fcntl() in the NFS case. This is what c-client does. Note that if you are going to use NFS to access files on a server which does not have statd/lockd running, your only choice is (3), (4), or (5). Here again, IMAP can bail you out. These problems aren't unique to c-client applications; they have also been reported with Elm, Mediamail, and other email tools. Of the other two SVR4 locking bugs: Programmer awareness is necessary to deal with the bug that you can not get an exclusive lock unless the file is open for write. I believe that c-client has fixed all of these cases. The problem about opening a second designator smashing any current locks on the file has not been addressed satisfactorily yet. This is not an easy problem to deal with, especially in c-client which really doesn't know what other files/streams may be open by Pine. Aren't you so happy that you bought an System V system?
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