runtime/cgo: avoid races on cgo_context_function

Change-Id: Ie9e6fda675e560234e90b9022526fd689d770818
Reviewed-on: https://go-review.googlesource.com/23610Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/cgo/gcc_context.c

@@ -7,21 +7,15 @@
 
 #include "libcgo.h"
 
-// The context function, used when tracing back C calls into Go.
-void (*x_cgo_context_function)(struct context_arg*);
-
-// Sets the context function to call to record the traceback context
-// when calling a Go function from C code. Called from runtime.SetCgoTraceback.
-void x_cgo_set_context_function(void (*context)(struct context_arg*)) {
-	x_cgo_context_function = context;
-}
-
 // Releases the cgo traceback context.
 void _cgo_release_context(uintptr_t ctxt) {
-	if (ctxt != 0 && x_cgo_context_function != nil) {
+	void (*pfn)(struct context_arg*);
+
+	pfn = _cgo_get_context_function();
+	if (ctxt != 0 && pfn != nil) {
 		struct context_arg arg;
 
 		arg.Context = ctxt;
-		(*x_cgo_context_function)(&arg);
+		(*pfn)(&arg);
 	}
 }

src/runtime/cgo/gcc_libinit.c

@@ -15,6 +15,9 @@ static pthread_cond_t runtime_init_cond = PTHREAD_COND_INITIALIZER;
 static pthread_mutex_t runtime_init_mu = PTHREAD_MUTEX_INITIALIZER;
 static int runtime_init_done;
 
+// The context function, used when tracing back C calls into Go.
+static void (*cgo_context_function)(struct context_arg*);
+
 void
 x_cgo_sys_thread_create(void* (*func)(void*), void* arg) {
 	pthread_t p;
@@ -27,16 +30,30 @@ x_cgo_sys_thread_create(void* (*func)(void*), void* arg) {
 
 uintptr_t
 _cgo_wait_runtime_init_done() {
+	void (*pfn)(struct context_arg*);
+
 	pthread_mutex_lock(&runtime_init_mu);
 	while (runtime_init_done == 0) {
 		pthread_cond_wait(&runtime_init_cond, &runtime_init_mu);
 	}
+
+	// TODO(iant): For the case of a new C thread calling into Go, such
+	// as when using -buildmode=c-archive, we know that Go runtime
+	// initialization is complete but we do not know that all Go init
+	// functions have been run. We should not fetch cgo_context_function
+	// until they have been, because that is where a call to
+	// SetCgoTraceback is likely to occur. We are going to wait for Go
+	// initialization to be complete anyhow, later, by waiting for
+	// main_init_done to be closed in cgocallbackg1. We should wait here
+	// instead. See also issue #15943.
+	pfn = cgo_context_function;
+
 	pthread_mutex_unlock(&runtime_init_mu);
-	if (x_cgo_context_function != nil) {
+	if (pfn != nil) {
 		struct context_arg arg;
 
 		arg.Context = 0;
-		(*x_cgo_context_function)(&arg);
+		(*pfn)(&arg);
 		return arg.Context;
 	}
 	return 0;
@@ -49,3 +66,21 @@ x_cgo_notify_runtime_init_done(void* dummy) {
 	pthread_cond_broadcast(&runtime_init_cond);
 	pthread_mutex_unlock(&runtime_init_mu);
 }
+
+// Sets the context function to call to record the traceback context
+// when calling a Go function from C code. Called from runtime.SetCgoTraceback.
+void x_cgo_set_context_function(void (*context)(struct context_arg*)) {
+	pthread_mutex_lock(&runtime_init_mu);
+	cgo_context_function = context;
+	pthread_mutex_unlock(&runtime_init_mu);
+}
+
+// Gets the context function.
+void (*(_cgo_get_context_function(void)))(struct context_arg*) {
+	void (*ret)(struct context_arg*);
+
+	pthread_mutex_lock(&runtime_init_mu);
+	ret = cgo_context_function;
+	pthread_mutex_unlock(&runtime_init_mu);
+	return ret;
+}

src/runtime/cgo/gcc_libinit_openbsd.c

@@ -6,6 +6,9 @@
 #include <stdlib.h>
 #include "libcgo.h"
 
+// The context function, used when tracing back C calls into Go.
+static void (*cgo_context_function)(struct context_arg*);
+
 void
 x_cgo_sys_thread_create(void* (*func)(void*), void* arg) {
 	fprintf(stderr, "x_cgo_sys_thread_create not implemented");
@@ -14,12 +17,16 @@ x_cgo_sys_thread_create(void* (*func)(void*), void* arg) {
 
 uintptr_t
 _cgo_wait_runtime_init_done() {
+	void (*pfn)(struct context_arg*);
+
 	// TODO(spetrovic): implement this method.
-	if (x_cgo_context_function != nil) {
+
+	pfn = _cgo_get_context_function();
+	if (pfn != nil) {
 		struct context_arg arg;
 
 		arg.Context = 0;
-		(*x_cgo_context_function)(&arg);
+		(*pfn)(&arg);
 		return arg.Context;
 	}
 	return 0;
@@ -29,3 +36,15 @@ void
 x_cgo_notify_runtime_init_done(void* dummy) {
 	// TODO(spetrovic): implement this method.
 }
+
+// Sets the context function to call to record the traceback context
+// when calling a Go function from C code. Called from runtime.SetCgoTraceback.
+void x_cgo_set_context_function(void (*context)(struct context_arg*)) {
+	// TODO(iant): Needs synchronization.
+	cgo_context_function = context;
+}
+
+// Gets the context function.
+void (*(_cgo_get_context_function(void)))(struct context_arg*) {
+	return cgo_context_function;
+}

src/runtime/cgo/gcc_libinit_windows.c

@@ -70,15 +70,18 @@ _cgo_is_runtime_initialized() {
 
 uintptr_t
 _cgo_wait_runtime_init_done() {
+	void (*pfn)(struct context_arg*);
+
 	 _cgo_maybe_run_preinit();
 	while (!_cgo_is_runtime_initialized()) {
 			WaitForSingleObject(runtime_init_wait, INFINITE);
 	}
-	if (x_cgo_context_function != nil) {
+	pfn = _cgo_get_context_function();
+	if (pfn != nil) {
 		struct context_arg arg;
 
 		arg.Context = 0;
-		(*x_cgo_context_function)(&arg);
+		(*pfn)(&arg);
 		return arg.Context;
 	}
 	return 0;
@@ -98,3 +101,23 @@ x_cgo_notify_runtime_init_done(void* dummy) {
 	}
 }
 
+// The context function, used when tracing back C calls into Go.
+static void (*cgo_context_function)(struct context_arg*);
+
+// Sets the context function to call to record the traceback context
+// when calling a Go function from C code. Called from runtime.SetCgoTraceback.
+void x_cgo_set_context_function(void (*context)(struct context_arg*)) {
+	EnterCriticalSection(&runtime_init_cs);
+	cgo_context_function = context;
+	LeaveCriticalSection(&runtime_init_cs);
+}
+
+// Gets the context function.
+void (*(_cgo_get_context_function(void)))(struct context_arg*) {
+	void (*ret)(struct context_arg*);
+
+	EnterCriticalSection(&runtime_init_cs);
+	ret = cgo_context_function;
+	LeaveCriticalSection(&runtime_init_cs);
+	return ret;
+}

src/runtime/cgo/libcgo.h

@@ -93,7 +93,7 @@ void darwin_arm_init_mach_exception_handler(void);
 struct context_arg {
 	uintptr_t Context;
 };
-extern void (*x_cgo_context_function)(struct context_arg*);
+extern void (*(_cgo_get_context_function(void)))(struct context_arg*);
 
 /*
  * TSAN support.  This is only useful when building with