SIMPLE SOLUTIONS

AEGIS LOCKS - reference manual online

How locking works, and which commands use them.

Chapter
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aegis locking(5)                                                                 aegis locking(5)

NAME aegis locks - how locking works, and which commands use them
DESCRIPTION Aegis maintains a database of information about the projects in its care, and the various changes, both completed and in progress. In order to ensure the integrity of this data‐ base, and also your project repository, it uses locks. From time to time, these locks are visible to the users, because they will be told that a command is waiting for a particular lock. For some transactions, this can be a long wait. Dining Philosophers While UNIX supplies locks in various flavors, if you need an entire set of locks simulta‐ neously, there is no elegant “all or nothing ” interface available. This is unsurpris‐ ing, as this is one of the classic computer science problems, known as the Dining Philosophers problem. The master lock is used to solve the Dining Philosophers problem, and is meant to be very transient. It is only held while the other locks which are required (frequently two or more, hence the problem) are requested - non-blocking. Once they are all obtained (or not, and any partials given back) the master lock is released. It is usually held for much less than a second. If you notice the master lock being held, it is almost always a symptom of the NFS lock daemon misbehaving. If the lock(s) could not be obtained, the blocking lock is waited on (without the mas‐ ter). This is when the "waiting for" message is issued. When obtained, it is released and the whole cycle starts again. This is why you occasionally see a series of "waiting for" messages. (This could maybe be optimized some, but it is still possible to block on yet another lock, and they you have to release all and wait again. As yet, I'm not con‐ vinced the extra code complexity is required.) Listing Locks There is a command available to list the current Aegis locks. aegis -list locks Note that the project names are change numbers are guesses as the locks are hashed over a 16-bit range, and range overlaps are possible. Collisions are also possible, but fortu‐ nately rarer. Known Problems There is a known problem with the HP/UX NFS clients. If you see persistent "no locks available" error messages when is NFS mounted, try making the /lockfile file world writable. chmod 666 /lockfile There is the possibility of a denial of service attack (which is why the default is 0600) but since you are presently denied service anyway, it's academic.
COMMANDS The following table shows the locks taken by the various commands. Note that theoreti‐ cally some of the commands take too few locks, but this has yet to prove to be a problem in practice. Also, "project state file" and "change state file" are the same thing for branches, it just depends which way you are looking at them at the time. Command Global Project Project Ancestor Change User State State Baseline Baselines State State File File File File ────────────────────────────────────────────────────────────────────────────────── aeb (dev) . . shared shared exclusive . aeb (int) . . . shared exclusive . aeca . . . . exclusive . aechown . . . . exclusive exclusive aeclean . . . . exclusive . aeclone . exclusive . . exclusive exclusive aecp . , , , exclusive . aecpu . . . . exclusive . aed . , , , exclusive . aedb . . . . exclusive exclusive aedbu . . . . exclusive exclusive aede . , . . exclusive exclusive aedeu . . . . exclusive exclusive aedn . exclusive . . . . aeib . exclusive . . exclusive exclusive aeibu . exclusive . . exclusive exclusive aeifail . exclusive . . exclusive exclusive aeipass . exclusive exclusive . exclusive exclusive aemv . , , , exclusive . aena . exclusive . . . . aenbr . exclusive . . . . aenbru exclusive exclusive . . . . aenc . exclusive . . . . aencu . exclusive . . . . aend . exclusive . . . . aenf . . . . exclusive . aenfu . . . . exclusive . aeni . exclusive . . . . aenpa exclusive . . . . . aenpr exclusive . . . . . aenrv . exclusive . . . . aent . exclusive . . exclusive . aentu . . . . exclusive . aepa . exclusive . . . . aera . exclusive . . . . aerd . exclusive . . . . aerfail . . . . exclusive exclusive aeri . exclusive . . . . aerm . , . . exclusive . aermu . . . . exclusive . aermpr exclusive . . . . . aerpa exclusive . . . . . aerpass . . . . exclusive . aerpu . . . . exclusive . aerrv . exclusive . . . . aet . . , , exclusive .
SEE ALSO You may wish to see the manual pages of all of the above commands. Many have descrip‐ tions of the locking interactions.
AUTHOR Peter Miller E-Mail: @canb.auug.org.au /\/\* WWW: http://www.canb.auug.org.au/~millerp/
Reference Manual Aegis aegis locking(5)