CODEWITHSUNDEEP
multithreadingconcurrencysynchronizationcompare-and-swap
gran33
gran33

Reputation: 12951

how to implement a new kind of compare&swap instruction

I need to implement (pseudo-code) a new type of compare&swap(a,b) (CAS) object (let's call the new type CAS2) .

CAS and CAS2 objects both support the read operation which returns the object value.

They both support the compare&swap(a,b) operation, but while on CAS this operation return true/false and changes the object value to b if it is equal to a, on CAS2 this operation has the same effect, but instead of returning true/false, it should always return the object value prior to the operation.

For example:

If the CAS object value is 4, compare&swap (4,5) will return true and change the value to 5, but on CAS2 object ,compare&swap (4,5) will return 4 and also change the value to 5 . If the CAS object value is 4, so compare&swap (5,6) will return false and will do nothing , but on CAS2 compare&swap (5,6) will return 4 and also do nothing. The CAS2 object should be implemented using only one CAS object and the implementation should be wait-free and linearizable.

Thanks in advance!

Upvotes: 1

Views: 181

Answers (1)

didierc
didierc

Reputation: 14730

The variant you are requesting is actually the mainsteam one. Implementing such an instruction without mutex requires hardware support, like a dedicated instruction. See the wikipedia article on that topic for further details.

Upvotes: 1

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