Luggage locks

In computer science, the fetch-and-add CPU instruction is a special instruction that atomically modifies the contents of a memory location. It is used to implement Mutual exclusion and concurrent algorithms in multiprocessor systems.

In uniprocessor systems, it is sufficient to disable interrupts before accessing a critical region.
However, in multiprocessor systems, it is impossible and undesirable to disable interrupts on all processors at the same time; and even with interrupts disabled two or more processors could be attempting to access the same memory at the same time. The fetch-and-add instruction, allows any processor to atomically increment a value in memory location, preventing such multiple processor collisions.

Maurice Herlihy (1993) proved that fetch-and-add is inferior to compare-and-swap.

Implementation

The standard fetch and add -instruction behaves like the following function. Crucially the entire function is executed atomically: no process can interrupt the function mid-execution and hence see a state that only exists during the execution of the function. This code only serves to help explain the behaviour of fetch-and-add; atomicity requires explicit hardware support and hence can not be implemented as a simple high level function.

<< atomic >>
function FetchAndAdd(address location) {
    int value := *location
    *location := value + 1
    return value
}

With fetch-and-add primitive a mutual exclusion lock can be implemented as:

 record locktype {
    int ticketnumber
    int turn
 }
 procedure LockInit( locktype* lock ) {
    lock.ticketnumber := 0
    lock.turn  := 0
 }
 procedure Lock( locktype* lock ) {
    int myturn := FetchAndAdd( &lock.ticketnumber )
    while lock.turn ≠ myturn
        skip // spin until lock is acquired
 }
 procedure UnLock( locktype* lock) {
    FetchAndAdd( &lock.turn )
 }

These routines provide a mutual-exclusion lock when following conditions are met:

• Locktype data structure is initialized with function LockInit before use
• Number of tasks waiting for the lock does not exceed INT_MAX at any time
• Integer datatype used in lock values can ‘wrap around’ when continuously incremented

x86 implementation

In the x86 architecture, the instruction ADD with the first operand specifying a memory location is a fetch-and-add instruction that has been there since the 8086 (it just wasn’t called that then), and with the LOCK prefix, is atomic across multiple processors. However, it could not return the original value of the memory location (though it returned some flags) until the 486 introduced the XADD instruction.

See also

• Test-and-set
  
• Test and Test-and-set
• Compare-and-swap
• Load-Link/Store-Conditional

• Howstuffworks "How Lock Picking Works" Lock picking allows people to open locks with only a few tools. Learn how to pick a lock, about types of keys and locks and about locksmith tools.
• Why lock(this) is bad - Bill Blogs in C# Well, the problem is one of how to manage the complexity involved with locks. When you say lock (this), the visibility of the object being locked matches
• Lock Videos Metacafe - Get the best internet videos - Funny videos, Amazing clips, Rare movies.
• EZ Lock Proposes a means of securing an occupied wheelchair in a moving vehicle.
• The Keyless Lock Store (The Access Control Experts!) Wholesaler of keyless lock products.
• Arrow Lock & Door Hardware Manufacturer of architectural hardware including locking hardware, door controls, closers, exit devices, exit alarm and key systems.
• Lock Haven University of Pennsylvania A member of the Pennsylvania State System of Higher Education.
• Quad-Lock Insulated Concrete Forms = Better Buildings Manufacturer of Insulating Concrete Forms (ICF) used in residential and commercial construction. Insulated Concrete Forms give you the best living
• Lavasoft Digital Lock - Lavasoft With convenient file selection and easy-to-use functions, Lavasoft Digital Lock is a natural addition to the privacy tools you use regularly to ensure your
• Home - Welcome to the Baby Lock Web Site For over 40 years, Baby Lock has been dedicated to the love of sewing by creating machines for sewing, embroidery, quilting and serging - all with
• Lock, Stock, & Barrel Your complete supplier of reloading, shooting, and gun supplies for the American sportsman. Full-line supplier of Hornady, Redding, RCBS, Leupold,
• Lock of Che's hair sold at Dallas auction | Oddly Enough | Reuters DALLAS (Reuters) - A lock of socialist revolutionary Ernesto Che Guevara's hair and related items were auctioned on Thursday in Dallas to a Houston-area
• Google Research Publication: Chubby Distributed Lock Service We describe our experiences with the Chubby lock service, which is intended to provide coarse-grained locking as well as reliable (though low-volume)
• Lock On: Modern Air Combat for PC - Lock On: Modern Air Combat PC Lock On: Modern Air Combat for PC - GameSpot offers reviews, previews, cheats, and more. Count on us for all of the latest on the Lock On: Modern Air Combat

Tags: Uncategorized //