Suppose all threads have passed through the atomic region. One thread then sets b->counter = 0 and b->flag = 1. This thread moves on to the next Barrier, takes the lock, and sets b->flag = 0. Then the other threads still in the barrier will become stuck in their while loop, and will never reach the second barrier.
The code sequence on the right causes deadlock:
Suppose all threads have passed through the atomic region. One thread then sets b->counter = 0 and b->flag = 1. This thread moves on to the next Barrier, takes the lock, and sets b->flag = 0. Then the other threads still in the barrier will become stuck in their while loop, and will never reach the second barrier.
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