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In computer science, read-modify-write is a class of atomic operations (such as test-and-set, fetch-and-add, and compare-and-swap) that both read a memory location and write a new value into it simultaneously, either with a completely new value or some function of the previous value. These operations prevent race conditions in multi-threaded applications. Typically they are used to implement mutexes or semaphores. These atomic operations are also heavily used in non-blocking synchronization.

Maurice Herlihy (1991) ranks atomic operations by their consensus numbers, as follows:

  • : memory-to-memory move and swap, augmented queue, compare-and-swap, fetch-and-cons, sticky byte, load-link/store-conditional (LL/SC)[1]
  • 2n - 2: n-register assignment
  • 2: test-and-set, swap, fetch-and-add, queue, stack
  • 1: atomic read and atomic write

It is impossible to implement an operation that requires a given consensus number with only operations with a lower consensus number, no matter how many of such operations one uses.[2] Read-modify-write instructions often produce unexpected results when used on I/O devices, as a write operation may not affect the same internal register that would be accessed in a read operation.[3]

This term is also associated with RAID levels that perform actual write operations as atomic read-modify-write sequences.[4] Such RAID levels include RAID 4, RAID 5 and RAID 6.

See also


  1. "Writing Lock-Free Code: A Corrected Queue" by Herb Sutter: "Compare-and-swap (CAS) is ... widely available ... However, some systems instead provide the equivalently powerful load-linked/store-conditional (LL/SC) instead."
  2. Herlihy, Maurice (January 1991). "Wait-free synchronization". ACM Trans. Program. Lang. Syst. 13 (1): 124–149. doi:10.1145/114005.102808. Retrieved 2007-05-20. 
  3. Massmind: "The read–modify–write problem"
  4. "Basic RAID Organizations". Retrieved 2013-10-04. 

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