From bed7e3ed78692b06128194abff79bd8c75731c88 Mon Sep 17 00:00:00 2001
From: Russ Cox <rsc@golang.org>
Date: Thu, 28 Jul 2011 21:03:40 -0400
Subject: [PATCH] gc: fix pprof deadlock

Fixes #2051.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4834041
---
 src/pkg/runtime/cpuprof.c |  4 ++++
 src/pkg/runtime/symtab.c  | 17 +++++++++++++----
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/src/pkg/runtime/cpuprof.c b/src/pkg/runtime/cpuprof.c
index 6233bcb457..74b795b7ee 100644
--- a/src/pkg/runtime/cpuprof.c
+++ b/src/pkg/runtime/cpuprof.c
@@ -121,6 +121,10 @@ 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)
diff --git a/src/pkg/runtime/symtab.c b/src/pkg/runtime/symtab.c
index ffa042e6f7..63e6d87849 100644
--- a/src/pkg/runtime/symtab.c
+++ b/src/pkg/runtime/symtab.c
@@ -420,10 +420,19 @@ runtime·findfunc(uintptr addr)
 	Func *f;
 	int32 nf, n;
 
-	runtime·lock(&funclock);
-	if(func == nil)
-		buildfuncs();
-	runtime·unlock(&funclock);
+	// 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;