Dekker’s algorithm is one of the most basic algorithms for mutual exclusion, alongside Peterson’s algorithm and Lamport’s bakery algorithm. Dekker’s algorithm (Q) frwiki Algorithme de Dekker; hywiki Դեկկերի ալգորիթմ; idwiki Algoritme dekker; itwiki Algoritmo di Dekker; jawiki デッカーの. (4) 48 (), – A. Dijksma, H. Langer and H. de Snoo, Characteristic functions of unitary operator colligations in IIk , Marcel Dekker, , pp.

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If the other process has not flagged intent, the critical section can be entered alforithme irrespective of the current turn. In particular, threads may continue to see “stale” values for some time, provided they see values in an order consistent with the constraints.

Steps between consecutive labels p1, It is possible that more than one thread will get the same number n when they request it; this cannot be avoided.

Or just one-shot acquisition by T1 and T2? In the example being discussed, we “know” that the fence in p0 occurs before the fence in p1. Submit to Reddit Submit to DZone.

The necessity of the variable Entering might not be obvious as there is no ‘lock’ around lines 7 to Retrieved from ” https: This section does not cite any sources. Processes indicate an intention to enter the critical section which is tested by the outer while loop.

I missed the fence after the store in the contention path. At least to me, that’s intuitive behavior. If p0 reads true for flag1it will enter the while loop rather than the critical section. It would be entirely possible for p0 to run to completion before p1’s store to flag1 became visible.

I really think a simple example like this would demonstrate the differences between one-sided fences across a pair of threads and two-sided fences on a single thread and why both models are necessary depending on whether you need the StoreLoad.

One disadvantage is that it is limited to two processes and makes use of busy waiting instead of process suspension. In real applications, different threads often have different “main” functions. For example, in C or Java, one would annotate these variables as ‘volatile’.

But I thought the standard did not allow it. However there is no such fence on contention path. A simple example is: That customer must draw another number from the numbering machine in order to shop again.


Modern operating systems provide mutual exclusion primitives that are more general and flexible than Dekker’s algorithm. I always thought this was a terrible gap in the standard that only makes composing SC and no-SC code impossible to reason about. When implementing the pseudo code in a single process system or under cooperative multitaskingit is better to replace the “do nothing” sections with code that notifies the operating system to immediately switch to the next thread.

Please help improve this section by adding citations to reliable sources.

Lamport’s bakery algorithm

So p1 will store its value and the fence will operate on the store buffer to update the value on thread 0 p0 cache. Lamport’s bakery algorithm is one of many mutual exclusion algorithms designed to prevent concurrent threads entering critical sections of code concurrently to eliminate the risk of data corruption.

For that matter, I could mark the loads and store here with acquire-release respectively and the result would still be the same.

Since the loop conditions will evaluate as falsethis does not cause much delay.

Because p1 set flag1 to falseeventually p0 will read flag1 as false and exit the outer while loop to enter the critical section. That’s how it should work. In the bakery analogy, it algorihme when the customer trades with the baker that others must wait. They both set their respective flags to trueexecute the fence and then read the other flag at the start of the while loop. Due to the limitations of computer architecture, some parts of Lamport’s analogy need slight modification.

If one process is already in the critical section, the other process will busy wait for the first process to exit.

Dekker’s algorithm

This page was last edited on 30 Decemberat Dekker’s algorityme is the first known correct solution to the mutual exclusion problem in concurrent programming. Alternatively, ordering can be guaranteed by the explicit use of separate fences, with the load and store operations using a relaxed ordering.

When p0 exits the critical section it sets turn to 1. If p0 reads the value of false for flag1it will not enter the while loop, and will instead enter the critical section, but that is OK since p1 has entered the while loop. If you’re like me then you’ll be interested in why stuff works, rather than just taking the code. In case another thread has the same number, the thread dek,er the smallest i will enter the critical section first.


All other customers must wait in a queue until the baker finishes serving the current customer and the next number is displayed.

Dekker’s algorithm – Wikipedia

algoeithme However, in the absence of actual contention between the two processes, the entry and exit from critical section is extremely efficient when Dekker’s algorithm is used. On the other side, there is no such guarantee for the read from flag1 in p0so p0 may or may not enter the while loop. In Lamport’s original paper, the entering variable is known as choosingand the following conditions apply:.

It is remarkable that this algorithm is not built on top of some lower level “atomic” operation, e. Hello, I know this post is a little old but i hope you will answer some of my question: It xe some thread-specific algorrithme that doesn’t algorith,e with other alglrithme resources and execution. Though flag0 is not set to false until p0 has finished the critical section, we need to ensure that p1 does not see this until the values modified in the critical section are also visible to p1and this is the purpose of the release fence prior to the store to flag0 and the acquire fence after the while loop.

Please help to improve this article by introducing more precise citations. One can verify this with cppmem with this code snippet: According to the analogy, the “customers” are threads, identified by the letter iobtained from a global variable. Therefore the read from flag0 must see the value stored trueso p1 will enter the while loop. The critical section is that part of code that requires exclusive access to resources and may only be executed by one thread at a time.