cmd/gc: do not race instrument syscall.forkAndExecInChild

Race instrumentation can allocate, switch stacks, preempt, etc.
All that is not allowed in between fork and exec.
Fixes #4840.

R=golang-dev, daniel.morsing, dave
CC=golang-dev
https://golang.org/cl/11324044

src/cmd/gc/racewalk.c

@@ -51,6 +51,18 @@ ispkgin(const char **pkgs, int n)
 	return 0;
 }
 
+static int
+isforkfunc(Node *fn)
+{
+	// Special case for syscall.forkAndExecInChild.
+	// In the child, this function must not acquire any locks, because
+	// they might have been locked at the time of the fork.  This means
+	// no rescheduling, no malloc calls, and no new stack segments.
+	// Race instrumentation does all of the above.
+	return myimportpath != nil && strcmp(myimportpath, "syscall") == 0 &&
+		strcmp(fn->nname->sym->name, "forkAndExecInChild") == 0;
+}
+
 void
 racewalk(Node *fn)
 {
@@ -58,7 +70,7 @@ racewalk(Node *fn)
 	Node *nodpc;
 	char s[1024];
 
-	if(ispkgin(omit_pkgs, nelem(omit_pkgs)))
+	if(ispkgin(omit_pkgs, nelem(omit_pkgs)) || isforkfunc(fn))
 		return;
 
 	if(!ispkgin(noinst_pkgs, nelem(noinst_pkgs))) {

src/pkg/syscall/exec_bsd.go

@@ -27,6 +27,7 @@ type SysProcAttr struct {
 // In the child, this function must not acquire any locks, because
 // they might have been locked at the time of the fork.  This means
 // no rescheduling, no malloc calls, and no new stack segments.
+// For the same reason compiler does not race instrument it.
 // The calls to RawSyscall are okay because they are assembly
 // functions that do not grow the stack.
 func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr *ProcAttr, sys *SysProcAttr, pipe int) (pid int, err Errno) {

src/pkg/syscall/exec_linux.go

@@ -28,6 +28,7 @@ type SysProcAttr struct {
 // In the child, this function must not acquire any locks, because
 // they might have been locked at the time of the fork.  This means
 // no rescheduling, no malloc calls, and no new stack segments.
+// For the same reason compiler does not race instrument it.
 // The calls to RawSyscall are okay because they are assembly
 // functions that do not grow the stack.
 func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr *ProcAttr, sys *SysProcAttr, pipe int) (pid int, err Errno) {