runtime: fix spurious deadlock reporting

Fixes #2337.
Unfortunate sequence of events is:
1. maxcpu=2, mcpu=1, grunning=1
2. starttheworld creates an extra M:
   maxcpu=2, mcpu=2, grunning=1
4. the goroutine calls runtime.GOMAXPROCS(1)
   maxcpu=1, mcpu=2, grunning=1
5. since it sees mcpu>maxcpu, it calls gosched()
6. schedule() deschedules the goroutine:
   maxcpu=1, mcpu=1, grunning=0
7. schedule() call getnextandunlock() which
   fails to pick up the goroutine again,
   because canaddcpu() fails, because mcpu==maxcpu
8. then it sees that grunning==0,
   reports deadlock and terminates

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5191044

src/pkg/runtime/malloc.h

@@ -407,7 +407,7 @@ enum
 
 void	runtime·MProf_Malloc(void*, uintptr);
 void	runtime·MProf_Free(void*, uintptr);
-int32	runtime·helpgc(void);
+int32	runtime·helpgc(bool*);
 void	runtime·gchelper(void);
 
 // Malloc profiling settings.

src/pkg/runtime/mgc0.c

@@ -916,10 +916,11 @@ runtime·gc(int32 force)
 	runtime·lock(&work.markgate);
 	runtime·lock(&work.sweepgate);
 
+	extra = false;
 	work.nproc = 1;
 	if(runtime·gomaxprocs > 1 && runtime·ncpu > 1) {
 		runtime·noteclear(&work.alldone);
-		work.nproc += runtime·helpgc();
+		work.nproc += runtime·helpgc(&extra);
 	}
 	work.nwait = 0;
 	work.ndone = 0;
@@ -984,7 +985,6 @@ runtime·gc(int32 force)
 	// coordinate.  This lazy approach works out in practice:
 	// we don't mind if the first couple gc rounds don't have quite
 	// the maximum number of procs.
-	extra = work.nproc < runtime·gomaxprocs && work.nproc < runtime·ncpu && work.nproc < MaxGcproc;
 	runtime·starttheworld(extra);
 
 	// give the queued finalizers, if any, a chance to run
@@ -1008,7 +1008,7 @@ runtime·UpdateMemStats(void)
 	cachestats();
 	m->gcing = 0;
 	runtime·semrelease(&gcsema);
-	runtime·starttheworld(0);
+	runtime·starttheworld(false);
 }
 
 static void

src/pkg/runtime/proc.c

@@ -602,9 +602,9 @@ top:
 }
 
 int32
-runtime·helpgc(void)
+runtime·helpgc(bool *extra)
 {
-	M *m;
+	M *mp;
 	int32 n, max;
 
 	// Figure out how many CPUs to use.
@@ -621,13 +621,15 @@ runtime·helpgc(void)
 
 	runtime·lock(&runtime·sched);
 	n = 0;
-	while(n < max && (m = mget(nil)) != nil) {
+	while(n < max && (mp = mget(nil)) != nil) {
 		n++;
-		m->helpgc = 1;
-		m->waitnextg = 0;
-		runtime·notewakeup(&m->havenextg);
+		mp->helpgc = 1;
+		mp->waitnextg = 0;
+		runtime·notewakeup(&mp->havenextg);
 	}
 	runtime·unlock(&runtime·sched);
+	if(extra)
+		*extra = n != max;
 	return n;
 }
 
@@ -685,9 +687,10 @@ runtime·starttheworld(bool extra)
 		// initialization work so is definitely running),
 		// but m is not running a specific goroutine,
 		// so set the helpgc flag as a signal to m's
-		// first schedule(nil) to mcpu--.
+		// first schedule(nil) to mcpu-- and grunning--.
 		m = startm();
 		m->helpgc = 1;
+		runtime·sched.grunning++;
 	}
 	schedunlock();
 }
@@ -833,6 +836,8 @@ schedule(G *gp)
 		v = runtime·xadd(&runtime·sched.atomic, -1<<mcpuShift);
 		if(atomic_mcpu(v) > maxgomaxprocs)
 			runtime·throw("negative mcpu in scheduler");
+		// Compensate for increment in starttheworld().
+		runtime·sched.grunning--;
 		m->helpgc = 0;
 	}
 

test/fixedbugs/bug370.go

@@ -0,0 +1,18 @@
+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2011 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+// issue 2337
+// The program deadlocked.
+
+import "runtime"
+
+func main() {
+	runtime.GOMAXPROCS(2)
+	runtime.GC()
+	runtime.GOMAXPROCS(1)
+}