Protecting Shared Resources
In order to run safely on multiprocessor machines, access to shared resources (files, data structures etc.) must be serialized so that threads or processes do not attempt to modify the same resource at the same time. Atomic operations, spinlocks, critical sections, mutual exclusions ("mutexes"), and serializing tokens are all possible methods that can be used to prevent concurrent access. Like recent versions of Linux and FreeBSD, XNU, as of Mac OS X 10.4 and Darwin 8.0, employs a fine-grained mutex model to achieve higher performance on multiprocessor systems.
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