The pthread_spin_unlock() function shall release the spin lock referenced
by lock which was locked via the pthread_spin_lock() or
pthread_spin_trylock() functions. The results are undefined if the lock
is not held by the calling thread. If there are threads spinning on the lock
when pthread_spin_unlock() is called, the lock becomes available and an
unspecified spinning thread shall acquire the lock.
The results are undefined if this function is called with an uninitialized
thread spin lock.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html