Editing Linearizability (section)

=== Compare-and-swap ===
{{Main|Compare-and-swap}}
Most systems provide an atomic compare-and-swap instruction that reads from a memory location, compares the value with an "expected" one provided by the user, and writes out a "new" value if the two match, returning whether the update succeeded. We can use this to fix the non-atomic counter algorithm as follows:

:# Read the value in the memory location;
:# add one to the value;
:# use compare-and-swap to write the incremented value back;
:# retry if the value read in by the compare-and-swap did not match the value we originally read.

Since the compare-and-swap occurs (or appears to occur) instantaneously, if another process updates the location while we are in-progress, the compare-and-swap is guaranteed to fail.