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Mutexes

Mutex locks (mutexes) ensure that at any moment in time, only one thread can access a shared object. Mutex is an amalgamation of mutual and exclusion.

Note

The foo lock described in the previous section was a mutex lock.

FreeBSD provides two types of mutex locks: spin mutexes and sleep mutexes.

Spin Mutexes

Spin mutexes are simple spin locks. If a thread attempts to acquire a spin lock that is being held by another thread, it will “spin” and wait for the lock to be released. Spin, in this case, means to loop infinitely on the CPU. This spinning can result in deadlock if a thread that is holding a spin lock is interrupted or if it context switches, and all subsequent threads attempt to acquire that lock. Consequently, while holding a spin mutex all interrupts are blocked on the local processor and a context switch cannot be performed.


  

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