Preempting a System Call

The kernel allows a thread to be preempted by a more favored thread, even when a system call is executing. This capability provides better system responsiveness for large multi-user systems.

Because system calls can be preempted, access to global data must be serialized. Kernel locking services, such as simple_lock and simple_unlock, are frequently used to serialize access to kernel data. A thread can be preempted even when it owns a lock. If multiple locks are obtained by system calls, a technique must be used to prevent multiple threads from deadlocking. One technique is to define a lock hierarchy. A system call must never return while holding a lock. For more information on locking, see Understanding Locking.