libthr: Support disable spinloop

Like yieldloops, we shoulde be able to set _thr_spinloops to zero.
Originally, it makes us to enformce default spin time even if we try to
disable it. Make MUTEX_ADAPTIVE_SPINS a one time initialization now.

Reviewed by:    kib
MFC after:      2 weeks
Differential Revision: https://reviews.freebsd.org/D60486

lib/libthr/thread/thr_init.c

@@ -118,7 +118,7 @@ size_t		_thr_guard_default;
 size_t		_thr_stack_default = THR_STACK_DEFAULT;
 size_t		_thr_stack_initial = THR_STACK_INITIAL;
 int		_thr_page_size;
-int		_thr_spinloops;
+int		_thr_spinloops = MUTEX_ADAPTIVE_SPINS;
 int		_thr_yieldloops;
 int		_thr_queuefifo = 4;
 int		_gc_count;

lib/libthr/thread/thr_mutex.c

@@ -62,12 +62,6 @@ _Static_assert(offsetof(struct pthread_mutex, m_qe) >= CACHE_LINE_SIZE,
     "m_qe must not share a cache line with m_lock.m_owner");
 #endif
 
-/*
- * For adaptive mutexes, how many times to spin doing trylock2
- * before entering the kernel to block
- */
-#define MUTEX_ADAPTIVE_SPINS	2000
-
 /*
  * Prototypes
  */
@@ -274,8 +268,7 @@ mutex_init_body(struct pthread_mutex *pmutex,
 		pmutex->m_lock.m_flags |= UMUTEX_ROBUST;
 	}
 	if (PMUTEX_TYPE(pmutex->m_flags) == PTHREAD_MUTEX_ADAPTIVE_NP) {
-		pmutex->m_spinloops =
-		    _thr_spinloops ? _thr_spinloops: MUTEX_ADAPTIVE_SPINS;
+		pmutex->m_spinloops = _thr_spinloops;
 		pmutex->m_yieldloops = _thr_yieldloops;
 	}
 }

lib/libthr/thread/thr_private.h

@@ -154,6 +154,12 @@ TAILQ_HEAD(mutex_queue, pthread_mutex);
 
 #define MAX_DEFER_WAITERS       50
 
+/*
+ * For adaptive mutexes, how many times to spin doing trylock2
+ * before entering the kernel to block
+ */
+#define MUTEX_ADAPTIVE_SPINS	2000
+
 /*
  * Values for pthread_mutex m_ps indicator.
  */