wait_var_event()

One of the trickiest aspects to concurrency in the kernel is waiting for a specific event to take place. There is a wide variety of possible events, including a process exiting, the last reference to a data structure going away, a device completing an operation, or a timeout occurring. Waiting is surprisingly hard to get right — race conditions abound to trap the unwary — so the kernel has accumulated a large set of wait_event_*() macros to make the task easier. An attempt to add a new one, though, has led to the generalization of specific types of waits for 4.17.

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