Enhance spin_adaptive() to yield after several iterations.

This avoids worst case behavior if e.g. another thread is preempted
while owning the resource the spinning thread is waiting for.

Makefile.in

@@ -189,6 +189,7 @@ TESTS_UNIT := \
 	$(srcroot)test/unit/size_classes.c \
 	$(srcroot)test/unit/slab.c \
 	$(srcroot)test/unit/smoothstep.c \
+	$(srcroot)test/unit/spin.c \
 	$(srcroot)test/unit/stats.c \
 	$(srcroot)test/unit/stats_print.c \
 	$(srcroot)test/unit/ticker.c \

include/jemalloc/internal/spin_inlines.h

@@ -8,14 +8,19 @@ void	spin_adaptive(spin_t *spin);
 #if (defined(JEMALLOC_ENABLE_INLINE) || defined(JEMALLOC_SPIN_C_))
 JEMALLOC_INLINE void
 spin_adaptive(spin_t *spin) {
-	volatile uint64_t i;
+	volatile uint32_t i;
 
-	for (i = 0; i < (KQU(1) << spin->iteration); i++) {
+	if (spin->iteration < 5) {
+		for (i = 0; i < (1U << spin->iteration); i++) {
 			CPU_SPINWAIT;
 		}
-
-	if (spin->iteration < 63) {
 		spin->iteration++;
+	} else {
+#ifdef _WIN32
+		SwitchToThread();
+#else
+		sched_yield();
+#endif
 	}
 }
 

test/unit/spin.c

@@ -0,0 +1,16 @@
+#include "test/jemalloc_test.h"
+
+TEST_BEGIN(test_spin) {
+	spin_t spinner = SPIN_INITIALIZER;
+
+	for (unsigned i = 0; i < 100; i++) {
+		spin_adaptive(&spinner);
+	}
+}
+TEST_END
+
+int
+main(void) {
+	return test(
+	    test_spin);
+}