In computer science, compare-and-swap (CAS) is an atomic CPU instruction used in multithreading to achieve synchronization. It compares the contents of a memory location to a given value and, only if they are the same, modifies the contents of that memory location to a given new value. The atomicity guarantees that the new value is calculated based on up-to-date information; if the value had been updated by another thread in the meantime, the write would fail. The result of the operation must indicate whether it performed the substitution; this can be done either with a simple Boolean response (this variant is often called compare-and-set), or by returning the value read from the memory location (not the value written to it).
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Compare-and-Swap (and Compare-and-Swap-Double) has been an integral part of the IBM 370 (and all successor) architectures since 1970. The operating systems which run on these architectures make extensive use of this instruction to facilitate process (i.e., system and user tasks) and processor (i.e., central processors) parallelism while eliminating, to the greatest degree possible, the "disabled spin locks" which were employed in earlier IBM operating systems. In these operating systems, new units of work may be instantiated "globally", into the Global Service Priority List, or "locally", into the Local Service Priority List, by the execution of a single Compare-and-Swap instruction. This dramatically improved the responsiveness of these operating systems.
In the x86 and Itanium architectures this is the compare and exchange (CMPXCHG) instruction, though here the lock prefix should be there to make it really atomic.
The following C function shows the basic behavior of a compare-and-swap variant that returns the old value of the specified memory location; however, this version does not provide the crucial guarantees of atomicity that a real compare-and-swap operation would:
int compare_and_swap (int* reg, int oldval, int newval) { int old_reg_val = *reg; if (old_reg_val == oldval) *reg = newval; return old_reg_val; }
old_reg_val is always returned, but it can be tested following the compare_and_swap operation to see if it matches oldval, as it may be different, meaning that another process has managed to succeed in a competing compare_and_swap to change the reg value from oldval.
For example, an election protocol can be done, where every process returns the result of compare_and_swap with its say PID (=newval), except the process which finds the compare_and_swap returned the initial non-pid value (e.g. zero), in which case it returns its own PID.
bool compare_and_swap (int *accum, int *dest, int newval) { if ( *accum == *dest ) { *dest = newval; return true; } else { *accum = *dest; return false; } }
This is the logic in the Intel Software Manual Vol 2A.
CAS is used to implement synchronization primitives like semaphores and mutexes, as well as more sophisticated lock-free and wait-free algorithms. Maurice Herlihy (1991) proved that CAS can implement more of these algorithms than atomic read, write, and fetch-and-add, and that, assuming a fairly large amount of memory, it can implement all of them.[1]
Algorithms built around CAS typically read some key memory location and remember the old value. Based on that old value, they compute some new value. Then they try to swap in the new value using CAS, where the comparison checks for the location still being equal to the old value. If CAS indicates that the attempt has failed, it has to be repeated from the beginning: the location is re-read, a new value is re-computed and the CAS is tried again.
Some of these algorithms are affected by and must handle the problem of a false positive match, or the ABA problem. It's possible that between the time the old value is read and the time CAS is attempted, some other processors or threads change the memory location two or more times such that it acquires a bit pattern which matches the old value. The problem arises if this new bit pattern, which looks exactly like the old value, has a different meaning: for instance, it could be a recycled address, or a wrapped version counter.
A general solution to this is to use a double-length CAS (e.g. on a 32 bit system, a 64 bit CAS). The second half is used to hold a counter. The compare part of the operation compares the previously read value of the pointer *and* the counter, to the current pointer and counter. If they match, the swap occurs - the new value is written - but the new value has an incremented counter. This means that if ABA has occurred, although the pointer value will be the same, the counter is exceedingly unlikely to be the same (for a 32 bit value, a multiple of 2^32 operations would have had to occurred, causing the counter to wrap and at that moment, the pointer value would have to also by chance be the same).
An alternative form of this (useful on CPUs which lack DCAS) is to use an index into a freelist, rather than a full pointer, e.g. with a 32 bit CAS, use a 16 bit index and a 16 bit counter. However, the reduced counter lengths begin to make ABA - especially at modern CPU speeds - likely.
One simple technique which helps alleviate this problem is to store an ABA counter in each data structure element, rather than using a single ABA counter for the whole data structure.
A more complicated but more effective solution is to implement SMR, Safe Memory Reclamation. This is in effect lock-free garbage collection. The upshot of using SMR is that you can know that a given pointer will only be present once at any one time in the data structure, so the ABA problem is completely solved. (Without SMR, something like a freelist will be in use, to ensure that all data elements can be accessed safely (no memory access violations) even when they are no longer present in the data structure. With SMR, only elements actually currently in the data structure will be accessed).
CAS, and other atomic instructions, are sometimes thought to be unnecessary in uniprocessor systems, because the atomicity of any sequence of instructions can be achieved by disabling interrupts while executing it. However, disabling interrupts has numerous downsides. For example, code that is allowed to do so must be trusted not to be malicious and monopolize the CPU, as well as to be correct and not accidentally hang the machine in an infinite loop. Further, disabling interrupts is often deemed too expensive to be practical. Thus, even programs only intended to run on uniprocessor machines will benefit from atomic instructions, as in the case of Linux's futexes.
In multiprocessor systems, it is usually impossible to disable interrupts on all processors at the same time. Even if it were possible, two or more processors could be attempting to access the same semaphore's memory at the same time, and thus atomicity would not be achieved. The compare-and-swap instruction allows any processor to atomically test and modify a memory location, preventing such multiple-processor collisions.
Since CAS operates on a single pointer-sized memory location, while most lock-free and wait-free algorithms need to modify multiple locations, several extensions have been implemented.
java.util.concurrent.atomicAtomicReferenceFieldUpdater.compareAndSet(T, V, V)