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If two threads try to access a critical section using lock, the race condition can cause both threads to enter the critical section at the same time. For example, if thread 1 performs the ld instruction and then thread 2 performs the ld instruction immediately after, both threads will have R0 as 0 and so they will both go ahead into the critical section.


If we make the operation "if 0, store 1" an atomic operation. This can be correct.


@xiaowend: I think we need to make all the three operations ("load word into R0", "if 0" and "store 1") into one atomic operation to make the implementation correct.

The example is the same as @Arnie had mentioned: if p1 load the word into R0, then p2 load the word into R0, even the later "if 0, store 1" operation is atomic, p1 and p2 will both enter the critical section.


Remember: a data race occurs when two or more threads try to access the same memory location without the proper locking mechanisms, and at least one is attempting to write to that location. The set "data races" is a subset of "race conditions." A race condition occurs when multiple actors accessing some arbitrary shared resource aren't properly synchronized (resulting in different possible outputs of your program).