runtime: convert forcegc helper to Go

Also fix a bunch of bugs:
1. Accesses to last_gc must be atomic (it's int64).
2. last_gc still can be 0 during first checks in sysmon, check for 0.
3. forcegc.g can be unitialized when sysmon accesses it:
        forcegc.g is initialized by main goroutine (forcegc.g = newproc1(...)),
        and main goroutine is unsynchronized with both sysmon and forcegc goroutine.
        Initialize forcegc.g in the forcegc goroutine itself instead.

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rsc
https://golang.org/cl/136770043

1 files changed, 9 insertions, 0 deletions

diff --git a/src/pkg/runtime/runtime.h b/src/pkg/runtime/runtime.h
index 4dfc4f2c42..21ccb76b3c 100644
--- a/src/pkg/runtime/runtime.h
+++ b/src/pkg/runtime/runtime.h
@@ -92,6 +92,7 @@ typedef	struct	ParForThread	ParForThread;
 typedef	struct	CgoMal		CgoMal;
 typedef	struct	PollDesc	PollDesc;
 typedef	struct	DebugVars	DebugVars;
+typedef struct	ForceGCState	ForceGCState;
 
 /*
  * Per-CPU declaration.
@@ -572,6 +573,13 @@ struct DebugVars
 	int32	scavenge;
 };
 
+struct ForceGCState
+{
+	Mutex	lock;
+	G*	g;
+	uint32	idle;
+};
+
 extern bool runtime·precisestack;
 extern bool runtime·copystack;
 
@@ -774,6 +782,7 @@ extern	uint32	runtime·cpuid_edx;
 extern	DebugVars	runtime·debug;
 extern	uintptr	runtime·maxstacksize;
 extern	Note	runtime·signote;
+extern	ForceGCState	runtime·forcegc;
 
 /*
  * common functions and data