gc: fix pprof deadlock

Fixes #2051.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4834041

src/pkg/runtime/cpuprof.c

@@ -122,6 +122,10 @@ runtime·SetCPUProfileRate(int32 hz)
 	uintptr *p;
 	uintptr n;
 	
+	// Call findfunc now so that it won't have to
+	// build tables during the signal handler.
+	runtime·findfunc(0);
+
 	// Clamp hz to something reasonable.
 	if(hz < 0)
 		hz = 0;

src/pkg/runtime/symtab.c

@@ -420,10 +420,19 @@ runtime·findfunc(uintptr addr)
 	Func *f;
 	int32 nf, n;
 
+	// Use atomic double-checked locking,
+	// because when called from pprof signal
+	// handler, findfunc must run without
+	// grabbing any locks.
+	// (Before enabling the signal handler,
+	// SetCPUProfileRate calls findfunc to trigger
+	// the initialization outside the handler.)
+	if(runtime·atomicloadp(&func) == nil) {
 		runtime·lock(&funclock);
 		if(func == nil)
 			buildfuncs();
 		runtime·unlock(&funclock);
+	}
 
 	if(nfunc == 0)
 		return nil;