runtime: refactor routines for stopping, running goroutine from m

This CL adds 'dropg', which is called to drop the association
between m and its current goroutine, and it makes schedule
handle locked goroutines correctly, instead of requiring all
callers of schedule to do that.

The effect is that if you want to take over an m for, say,
garbage collection work while still allowing the current g
to run on some other m, you can do an mcall to a function
that is:

        // dissociate gp
        dropg();
        gp->status = Gwaiting; // for ready

        // put gp on run queue for others to find
        runtime·ready(gp);

        /* ... do other work here ... */

        // done with m, let it run goroutines again
        schedule();

Before this CL, the dropg() body had to be written explicitly,
and the check for lockedg before schedule had to be
written explicitly too, both of which make the code a bit
more fragile than it needs to be.

LGTM=iant
R=dvyukov, iant
CC=golang-codereviews, rlh
https://golang.org/cl/113110043

src/pkg/runtime/proc.c

@@ -1320,6 +1320,11 @@ schedule(void)
 	if(g->m->locks)
 		runtime·throw("schedule: holding locks");
 
+	if(g->m->lockedg) {
+		stoplockedm();
+		execute(g->m->lockedg);  // Never returns.
+	}
+
 top:
 	if(runtime·sched.gcwaiting) {
 		gcstopm();
@@ -1360,6 +1365,22 @@ top:
 	execute(gp);
 }
 
+// dropg removes the association between m and the current goroutine m->curg (gp for short).
+// Typically a caller sets gp's status away from Grunning and then
+// immediately calls dropg to finish the job. The caller is also responsible
+// for arranging that gp will be restarted using runtime·ready at an
+// appropriate time. After calling dropg and arranging for gp to be
+// readied later, the caller can do other work but eventually should
+// call schedule to restart the scheduling of goroutines on this m.
+void
+dropg(void)
+{
+	if(g->m->lockedg == nil) {
+		g->m->curg->m = nil;
+		g->m->curg = nil;
+	}
+}
+
 // Puts the current goroutine into a waiting state and calls unlockf.
 // If unlockf returns false, the goroutine is resumed.
 void
@@ -1396,8 +1417,8 @@ park0(G *gp)
 	bool ok;
 
 	gp->status = Gwaiting;
-	gp->m = nil;
-	g->m->curg = nil;
+	dropg();
+
 	if(g->m->waitunlockf) {
 		ok = g->m->waitunlockf(gp, g->m->waitlock);
 		g->m->waitunlockf = nil;
@@ -1407,10 +1428,7 @@ park0(G *gp)
 			execute(gp);  // Schedule it back, never returns.
 		}
 	}
-	if(g->m->lockedg) {
-		stoplockedm();
-		execute(gp);  // Never returns.
-	}
+
 	schedule();
 }
 
@@ -1428,15 +1446,11 @@ void
 runtime·gosched0(G *gp)
 {
 	gp->status = Grunnable;
-	gp->m = nil;
-	g->m->curg = nil;
+	dropg();
 	runtime·lock(&runtime·sched);
 	globrunqput(gp);
 	runtime·unlock(&runtime·sched);
-	if(g->m->lockedg) {
-		stoplockedm();
-		execute(gp);  // Never returns.
-	}
+
 	schedule();
 }
 
@@ -1462,6 +1476,7 @@ goexit0(G *gp)
 	gp->status = Gdead;
 	gp->m = nil;
 	gp->lockedm = nil;
+	g->m->lockedg = nil;
 	gp->paniconfault = 0;
 	gp->defer = nil; // should be true already but just in case.
 	gp->panic = nil; // non-nil for Goexit during panic. points at stack-allocated data.
@@ -1469,8 +1484,9 @@ goexit0(G *gp)
 	gp->writebuf = nil;
 	gp->waitreason = nil;
 	gp->param = nil;
-	g->m->curg = nil;
-	g->m->lockedg = nil;
+	
+	dropg();
+
 	if(g->m->locked & ~LockExternal) {
 		runtime·printf("invalid m->locked = %d\n", g->m->locked);
 		runtime·throw("internal lockOSThread error");
@@ -1680,8 +1696,7 @@ exitsyscall0(G *gp)
 	P *p;
 
 	gp->status = Grunnable;
-	gp->m = nil;
-	g->m->curg = nil;
+	dropg();
 	runtime·lock(&runtime·sched);
 	p = pidleget();
 	if(p == nil)