Thursday, September 10, 2009

lockfree and compare-and-swap in my project

A simple race condition - If 2 threads update the same object in the same clock cycle, how do we avoid lost update?

Traditional solution is synchronization, i.e. locking. There's huge interest in lockfree, but I feel either it's impossible or requires hardware support like xchg or cmpxchg(?).

while (!done) {
final PositionMark oldMark = getRegion().get(key);
newMark = producer.producePrice(key);
final PositionMark replaced = getRegion().put(key, newMark); // atomic?
// The following ensures consistency given concurrent updates to the same key
done = (replaced == oldMark);
}

Idea is to first get old value of object, then update it and get the replaced value in one sweep. (Perhaps using xchg). If old != replaced, then keep retrying. This is probably swap-and-compare, not to be confused with the more complex compare-and-swap hardware feature [1].

Challenge is the atomicity of the xchg. Without hardware support, can u guarantee atomicity without locking? I doubt it.

In this code, getRegion returns a gemfire region, backed by a concurrent hash map, whose put() is actually synchronized with lock striping. If you were to use a hash map, I would assume put() is completely thread unsafe. 2 threads can both get the same replacedMark, and put in their new marks and overwrite each other -- lost update.

[1] compare-And-Swap does the swap conditionally -- safer.

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