runtime: changes to g->atomicstatus (nee status) to support concurrent GC

Every change to g->atomicstatus is now done atomically so that we can
ensure that all gs pass through a gc safepoint on demand. This allows
the GC to move from one phase to the next safely. In some phases the
stack will be scanned. This CL only deals with the infrastructure that
allows g->atomicstatus to go from one state to another. Future CLs
will deal with scanning and monitoring what phase the GC is in.

The major change was to moving to using a Gscan bit to indicate that
the status is in a scan state. The only bug fix was in oldstack where
I wasn't moving to a Gcopystack state in order to block scanning until
the new stack was in place. The proc.go file is waiting for an atomic
load instruction.

LGTM=rsc
R=golang-codereviews, dvyukov, josharian, rsc
CC=golang-codereviews, khr
https://golang.org/cl/132960044

1 files changed, 8 insertions, 6 deletions

diff --git a/src/pkg/runtime/heapdump.c b/src/pkg/runtime/heapdump.c
index 61f6fc2d95..ea299d0a2e 100644
--- a/src/pkg/runtime/heapdump.c
+++ b/src/pkg/runtime/heapdump.c
@@ -396,7 +396,7 @@ dumpgoroutine(G *gp)
 	dumpint((uintptr)sp);
 	dumpint(gp->goid);
 	dumpint(gp->gopc);
-	dumpint(gp->status);
+	dumpint(runtime·readgstatus(gp));
 	dumpbool(gp->issystem);
 	dumpbool(false);  // isbackground
 	dumpint(gp->waitsince);
@@ -442,14 +442,16 @@ dumpgs(void)
 {
 	G *gp;
 	uint32 i;
+	uint32 status;
 
 	// goroutines & stacks
 	for(i = 0; i < runtime·allglen; i++) {
 		gp = runtime·allg[i];
-		switch(gp->status){
+		status = runtime·readgstatus(gp); // The world is stopped so gp will not be in a scan state.
+		switch(status){
 		default:
-			runtime·printf("unexpected G.status %d\n", gp->status);
-			runtime·throw("mark - bad status");
+			runtime·printf("runtime: unexpected G.status %d\n", status);
+			runtime·throw("dumpgs in STW - bad status");
 		case Gdead:
 			break;
 		case Grunnable:
@@ -730,7 +732,7 @@ mdump(G *gp)
 	flush();
 
 	gp->param = nil;
-	gp->status = Grunning;
+	runtime·casgstatus(gp, Gwaiting, Grunning);
 	runtime·gogo(&gp->sched);
 }
 
@@ -751,7 +753,7 @@ runtime∕debug·WriteHeapDump(uintptr fd)
 	dumpfd = fd;
 
 	// Call dump routine on M stack.
-	g->status = Gwaiting;
+	runtime·casgstatus(g, Grunning, Gwaiting);
 	g->waitreason = runtime·gostringnocopy((byte*)"dumping heap");
 	runtime·mcall(mdump);