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Without special action in the deletion code, the search code would be prone to races with those deletions, described in Section 2.1. To handle such race conditions, the update side performs the update in two phases as follows: (1) remove permanent-variable pointers to the item being deleted, and (2) after a grace period has elapsed, free up the item's memory.

The grace period is not a fixed time duration, but is instead inferred by checking for per-CPU quiescent states, such as context switches in non-preemptive environments. Since kernel threads are prohibited from holding locks across a context switch, they also prohibited from holding pointers to data structures protected by those locks across context switches-after all, the entire data structure could well be deleted by some other CPU at any time this CPU does not hold the lock.

A trivial implementation of RCU in a non-preemptive kernel could simply declare the grace period over once it observed each CPU undergoing a context switch. Once the grace period completes, no CPU references the deleted item, and no CPU can gain such a reference. It is therefore safe to free up the deleted item.

With this approach, searches already in progress when the first phase executes might (or might not) see the item being deleted. However, searches that start after the first phase completes are guaranteed to never reference this item. Efficient mechanisms for determining the duration of the grace period are key to RCU, and are described fully elsewhere [McK02a]. These mechanisms track ``naturally occurring'' quiescent states, which removes the need for adding expensive context switches. In addition, these mechanisms take advantage of the fact that a single grace period can satisfy multiple concurrent updates, amortizing the cost of detecting a grace period over the updates.

next up previous
Next: RCU API Up: Background Previous: Glossary
Paul McKenney 2003-03-28