From de8a68e85304848189643fb48100c18aa9d60e32 Mon Sep 17 00:00:00 2001
From: Jason Evans <jasone@canonware.com>
Date: Wed, 8 Feb 2017 10:30:44 -0800
Subject: [PATCH] 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                              |  1 +
 include/jemalloc/internal/spin_inlines.h | 17 +++++++++++------
 test/unit/spin.c                         | 16 ++++++++++++++++
 3 files changed, 28 insertions(+), 6 deletions(-)
 create mode 100644 test/unit/spin.c

diff --git a/Makefile.in b/Makefile.in
index acd31f73..23056f78 100644
--- a/Makefile.in
+++ b/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 \
diff --git a/include/jemalloc/internal/spin_inlines.h b/include/jemalloc/internal/spin_inlines.h
index 03beeada..16573261 100644
--- a/include/jemalloc/internal/spin_inlines.h
+++ b/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++) {
-		CPU_SPINWAIT;
-	}
-
-	if (spin->iteration < 63) {
+	if (spin->iteration < 5) {
+		for (i = 0; i < (1U << spin->iteration); i++) {
+			CPU_SPINWAIT;
+		}
 		spin->iteration++;
+	} else {
+#ifdef _WIN32
+		SwitchToThread();
+#else
+		sched_yield();
+#endif
 	}
 }
 
diff --git a/test/unit/spin.c b/test/unit/spin.c
new file mode 100644
index 00000000..bd368b3d
--- /dev/null
+++ b/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);
+}