7.1.2.1 Right- and Left-Hand Locks
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7.1.2.1 Right- and Left-Hand Locks

Figure: Double-Ended Queue With Left- and Right-Hand Locks

One seemingly straightforward approach would be to have a left-hand lock for left-hand-end enqueue and dequeue operations along with a right-hand lock for right-hand-end operations, as shown in Figure . However, the problem with this approach is that the two locks' domains must overlap when there are fewer than four elements on the list. This overlap is due to the fact that removing any given element affects not only that element, but also its left- and right-hand neighbors. These domains are indicated by color in the figure, with blue indicating the domain of the left-hand lock, red indicating the domain of the right-hand lock, and purple indicating overlapping domains. Although it is possible to create an algorithm that works this way, the fact that it has no fewer than five special cases should raise a big red flag, especially given that concurrent activity at the other end of the list can shift the queue from one special case to another at any time. It is far better to consider other designs.