When multiple threads are updating ***isSame*** variable

isSame &= isSameTreeMultiThreaded(p.left, q.left, numThreads/2);
SIMILAR TO
isSame = isSame & isSameTreeMultiThreaded(p.left, q.left, numThreads/2);

1. It reads isSame which is thread safe gives latest value due to primitive
2. & operator with isSame variable ( one more, by the time it evaluates through recursive function, isSame variable could be outdated )
3. Updating isSame variable which is thread safe due to primitive

But, All the above 3 steps together is not atomic.

Consider the scenario:

isSame is true by default,

1. Thread-1 reads as true
2. Thread-2 reads as true
3. Thread-1 receives false from recursive function and updates it with false
4. Thread-2 receive true from recursive function and update it with true because it had already read isSame to be true

So, finally, we have true is saved in isSame variable which is wrong.

Volatile (Use cases):

1. Date races caused by shuffling of instructions by JAVA optimization based on happens-before relation. Volatile keyword, make sure all instructions before volatile variable is executed before it.
2. Partial reading of double, long which are 64-bit because update of 64-bit is a 2-step process.

My Solution: We should synchronize the update of isSame variable. With understanding, I did not see any use of volatile keyword here.

Note: My understanding could be partial, Since these programs were written by experts, there might be some rational behind it. I would like to know about it.

Hoping of quick reply.

Thank You,

Mahendra Reddy