runtime: fix scheduler race condition

In starttheworld() we assume that P's with local work
are situated in the beginning of idle P list.
However, once we start the first M, it can execute all local G's
and steal G's from other P's.
That breaks the assumption above. Thus starttheworld() will fail
to start some P's with local work.
It seems that it can not lead to very bad things, but still
it's wrong and breaks other assumtions
(e.g. we can have a spinning M with local work).
The fix is to collect all P's with local work first,
and only then start them.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/10051045

diff --git a/src/pkg/runtime/proc.c b/src/pkg/runtime/proc.c
index 432298a9ca2d5292f2315da237b8b19e2d3642af..5b3dbab7e05dd491a33a38c9d647f4188ecdd2ae 100644 (file)
--- a/src/pkg/runtime/proc.c
+++ b/src/pkg/runtime/proc.c
@@ -420,16 +420,9 @@ runtime·starttheworld(void)
                        pidleput(p);
                        break;
                }
-               mp = mget();
-               if(mp == nil) {
-                       p->link = p1;
-                       p1 = p;
-                       continue;
-               }
-               if(mp->nextp)
-                       runtime·throw("starttheworld: inconsistent mp->nextp");
-               mp->nextp = p;
-               runtime·notewakeup(&mp->park);
+               p->m = mget();
+               p->link = p1;
+               p1 = p;
        }
        if(runtime·sched.sysmonwait) {
                runtime·sched.sysmonwait = false;
@@ -440,8 +433,18 @@ runtime·starttheworld(void)
        while(p1) {
                p = p1;
                p1 = p1->link;
-               add = false;
-               newm(nil, p);
+               if(p->m) {
+                       mp = p->m;
+                       p->m = nil;
+                       if(mp->nextp)
+                               runtime·throw("starttheworld: inconsistent mp->nextp");
+                       mp->nextp = p;
+                       runtime·notewakeup(&mp->park);
+               } else {
+                       // Start M to run P.  Do not start another M below.
+                       newm(nil, p);
+                       add = false;
+               }
        }
 
        if(add) {
@@ -1154,6 +1157,8 @@ top:
        }
 
        gp = runqget(m->p);
+       if(gp && m->spinning)
+               runtime·throw("schedule: spinning with local work");
        if(gp == nil)
                gp = findrunnable();