diff --git a/benchmarks/stdio_benchmark.cpp b/benchmarks/stdio_benchmark.cpp
index fe25d7629..5658a506e 100644
--- a/benchmarks/stdio_benchmark.cpp
+++ b/benchmarks/stdio_benchmark.cpp
@@ -17,6 +17,7 @@
 #include "benchmark.h"
 
 #include <stdio.h>
+#include <stdio_ext.h>
 
 #define KB 1024
 #define MB 1024*KB
@@ -29,6 +30,7 @@ template <typename Fn>
 static void ReadWriteTest(int iters, int chunk_size, Fn f, bool buffered) {
   StopBenchmarkTiming();
   FILE* fp = fopen("/dev/zero", "rw");
+  __fsetlocking(fp, FSETLOCKING_BYCALLER);
   char* buf = new char[chunk_size];
   StartBenchmarkTiming();
 
@@ -66,12 +68,22 @@ static void BM_stdio_fwrite_unbuffered(int iters, int chunk_size) {
 }
 BENCHMARK(BM_stdio_fwrite_unbuffered)->AT_COMMON_SIZES;
 
-static void BM_stdio_fopen_fgets_fclose(int iters) {
+static void FopenFgetsFclose(int iters, bool no_locking) {
   char buf[1024];
   for (int i = 0; i < iters; ++i) {
     FILE* fp = fopen("/proc/version", "re");
+    if (no_locking) __fsetlocking(fp, FSETLOCKING_BYCALLER);
     fgets(buf, sizeof(buf), fp);
     fclose(fp);
   }
 }
-BENCHMARK(BM_stdio_fopen_fgets_fclose);
+
+static void BM_stdio_fopen_fgets_fclose_locking(int iters) {
+  FopenFgetsFclose(iters, false);
+}
+BENCHMARK(BM_stdio_fopen_fgets_fclose_locking);
+
+static void BM_stdio_fopen_fgets_fclose_no_locking(int iters) {
+  FopenFgetsFclose(iters, true);
+}
+BENCHMARK(BM_stdio_fopen_fgets_fclose_no_locking);
diff --git a/libc/stdio/fileext.h b/libc/stdio/fileext.h
index 25b7bda24..75230cda1 100644
--- a/libc/stdio/fileext.h
+++ b/libc/stdio/fileext.h
@@ -33,6 +33,7 @@
 #define _FILEEXT_H_
 
 #include <pthread.h>
+#include <stdbool.h>
 
 __BEGIN_DECLS
 
@@ -40,9 +41,10 @@ __BEGIN_DECLS
  * file extension
  */
 struct __sfileext {
-	struct	__sbuf _ub; /* ungetc buffer */
+	struct	__sbuf _ub;		/* ungetc buffer */
 	struct wchar_io_data _wcio;	/* wide char io status */
-	pthread_mutex_t _lock; /* file lock */
+	pthread_mutex_t _lock;		/* file lock */
+	bool _stdio_handles_locking;	/* __fsetlocking support */
 };
 
 #define _EXT(fp) ((struct __sfileext *)((fp)->_ext._base))
@@ -54,7 +56,8 @@ do { \
 	_UB(fp)._base = NULL; \
 	_UB(fp)._size = 0; \
 	WCIO_INIT(fp); \
-        _FLOCK(fp).value = __PTHREAD_RECURSIVE_MUTEX_INIT_VALUE; \
+	_FLOCK(fp).value = __PTHREAD_RECURSIVE_MUTEX_INIT_VALUE; \
+	_EXT(fp)->_stdio_handles_locking = true; \
 } while (0)
 
 #define _FILEEXT_SETUP(f, fext) \
diff --git a/libc/stdio/local.h b/libc/stdio/local.h
index 46b11f13a..7033eda9e 100644
--- a/libc/stdio/local.h
+++ b/libc/stdio/local.h
@@ -111,8 +111,8 @@ extern void __atexit_register_cleanup(void (*)(void));
 	(fp)->_lb._base = NULL; \
 }
 
-#define FLOCKFILE(fp)   flockfile(fp)
-#define FUNLOCKFILE(fp) funlockfile(fp)
+#define FLOCKFILE(fp)   if (_EXT(fp)->_stdio_handles_locking) flockfile(fp)
+#define FUNLOCKFILE(fp) if (_EXT(fp)->_stdio_handles_locking) funlockfile(fp)
 
 #define FLOATING_POINT
 #define PRINTF_WIDE_CHAR
diff --git a/libc/stdio/stdio_ext.cpp b/libc/stdio/stdio_ext.cpp
index bfdecb81c..fea44f68b 100644
--- a/libc/stdio/stdio_ext.cpp
+++ b/libc/stdio/stdio_ext.cpp
@@ -27,13 +27,10 @@
  */
 
 #include <stdio_ext.h>
+#include <stdlib.h>
 
-#include <stdio.h>
 #include "local.h"
-
-#define FSETLOCKING_QUERY 0
-#define FSETLOCKING_INTERNAL 1
-#define FSETLOCKING_BYCALLER 2
+#include "private/libc_logging.h"
 
 size_t __fbufsize(FILE* fp) {
   return fp->_bf._size;
@@ -76,11 +73,19 @@ void _flushlbf() {
   fflush(NULL);
 }
 
-int __fsetlocking(FILE*, int) {
-  // We don't currently have an implementation that would obey this,
-  // so make setting the state a no-op and always return "we handle locking for you".
-  // http://b/17154740 suggests ways we could fix this.
-  return FSETLOCKING_INTERNAL;
+int __fsetlocking(FILE* fp, int type) {
+  int old_state = _EXT(fp)->_stdio_handles_locking ? FSETLOCKING_INTERNAL : FSETLOCKING_BYCALLER;
+  if (type == FSETLOCKING_QUERY) {
+    return old_state;
+  }
+
+  if (type != FSETLOCKING_INTERNAL && type != FSETLOCKING_BYCALLER) {
+    // The API doesn't let us report an error, so blow up.
+    __libc_fatal("Bad type (%d) passed to __fsetlocking", type);
+  }
+
+  _EXT(fp)->_stdio_handles_locking = (type == FSETLOCKING_INTERNAL);
+  return old_state;
 }
 
 void clearerr_unlocked(FILE* fp) {
diff --git a/tests/stdio_ext_test.cpp b/tests/stdio_ext_test.cpp
index 950c7ed06..c95cbbdd5 100644
--- a/tests/stdio_ext_test.cpp
+++ b/tests/stdio_ext_test.cpp
@@ -133,7 +133,10 @@ TEST(stdio_ext, __freadable__fwritable) {
 
 TEST(stdio_ext, __fsetlocking) {
   FILE* fp = fopen("/proc/version", "r");
-  // Android doesn't actually support the other modes.
+  ASSERT_EQ(FSETLOCKING_INTERNAL, __fsetlocking(fp, FSETLOCKING_QUERY));
+  ASSERT_EQ(FSETLOCKING_INTERNAL, __fsetlocking(fp, FSETLOCKING_BYCALLER));
+  ASSERT_EQ(FSETLOCKING_BYCALLER, __fsetlocking(fp, FSETLOCKING_QUERY));
+  ASSERT_EQ(FSETLOCKING_BYCALLER, __fsetlocking(fp, FSETLOCKING_INTERNAL));
   ASSERT_EQ(FSETLOCKING_INTERNAL, __fsetlocking(fp, FSETLOCKING_QUERY));
   fclose(fp);
 }