Conflicting mutex(9) man page ?

[Paul Goyette <paul@xxxxxxxxxxxx>]

Man page for mutex(9) says

     mutex_exit(mtx)

           Release a mutex.  The mutex must have been previously
           acquired by the caller.  Mutexes may be released out of order
           as needed.

     ...

     mutex_spin_exit(mtx)

           Equivalent to mutex_exit(), but may only be used when it is
           known that mtx is a spin mutex.  On some architectures, this
           can substantially reduce the cost of releasing an unheld spin
           mutex.

The conflict:

If mutex_exit() requires that "the mutex must have been previously 
acquired by the caller", how can mutex_spin_exit() succeed when 
"releasing an unheld spin mutex"?


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