runtime: apply minor tweaks to channels

Remove complicated PRNG algorithm
(argument is limited by uint16 and can't be <= 1).
Do not require chansend/chanrecv selgen to be bumped with CAS.

R=rsc, ken
CC=golang-dev
https://golang.org/cl/4816041

diff --git a/src/pkg/runtime/chan.c b/src/pkg/runtime/chan.c
index 926bde723c2f75ad9ceef8bc8baeb4ec06f83828..0fdf771968dbe67d480aa60eef5459519dab338c 100644 (file)
--- a/src/pkg/runtime/chan.c
+++ b/src/pkg/runtime/chan.c
@@ -6,6 +6,7 @@
 #include "type.h"
 
 #define        MAXALIGN        7
+#define        NOSELGEN        1
 
 static int32   debug   = 0;
 
@@ -87,7 +88,6 @@ static        SudoG*  dequeue(WaitQ*, Hchan*);
 static void    enqueue(WaitQ*, SudoG*);
 static SudoG*  allocsg(Hchan*);
 static void    freesg(Hchan*, SudoG*);
-static uint32  fastrandn(uint32);
 static void    destroychan(Hchan*);
 
 Hchan*
@@ -215,7 +215,7 @@ runtime·chansend(Hchan *c, byte *ep, bool *pres)
 
        mysg.elem = ep;
        mysg.g = g;
-       mysg.selgen = g->selgen;
+       mysg.selgen = NOSELGEN;
        g->param = nil;
        g->status = Gwaiting;
        enqueue(&c->sendq, &mysg);
@@ -243,7 +243,7 @@ asynch:
                }
                mysg.g = g;
                mysg.elem = nil;
-               mysg.selgen = g->selgen;
+               mysg.selgen = NOSELGEN;
                g->status = Gwaiting;
                enqueue(&c->sendq, &mysg);
                runtime·unlock(c);
@@ -322,7 +322,7 @@ runtime·chanrecv(Hchan* c, byte *ep, bool *selected, bool *received)
 
        mysg.elem = ep;
        mysg.g = g;
-       mysg.selgen = g->selgen;
+       mysg.selgen = NOSELGEN;
        g->param = nil;
        g->status = Gwaiting;
        enqueue(&c->recvq, &mysg);
@@ -354,7 +354,7 @@ asynch:
                }
                mysg.g = g;
                mysg.elem = nil;
-               mysg.selgen = g->selgen;
+               mysg.selgen = NOSELGEN;
                g->status = Gwaiting;
                enqueue(&c->recvq, &mysg);
                runtime·unlock(c);
@@ -854,7 +854,7 @@ selectgo(Select **selp)
                sel->order[i] = i;
        for(i=1; i<sel->ncase; i++) {
                o = sel->order[i];
-               j = fastrandn(i+1);
+               j = runtime·fastrand1()%(i+1);
                sel->order[i] = sel->order[j];
                sel->order[j] = o;
        }
@@ -1151,7 +1151,9 @@ loop:
        q->first = sgp->link;
 
        // if sgp is stale, ignore it
-       if(!runtime·cas(&sgp->g->selgen, sgp->selgen, sgp->selgen + 1)) {
+       if(sgp->selgen != NOSELGEN &&
+               (sgp->selgen != sgp->g->selgen ||
+               !runtime·cas(&sgp->g->selgen, sgp->selgen, sgp->selgen + 2))) {
                //prints("INVALID PSEUDOG POINTER\n");
                freesg(c, sgp);
                goto loop;
@@ -1220,21 +1222,3 @@ freesg(Hchan *c, SudoG *sg)
                c->free = sg;
        }
 }
-
-static uint32
-fastrandn(uint32 n)
-{
-       uint32 max, r;
-
-       if(n <= 1)
-               return 0;
-
-       r = runtime·fastrand1();
-       if(r < (1ULL<<31)-n)  // avoid computing max in common case
-               return r%n;
-
-       max = (1ULL<<31)/n * n;
-       while(r >= max)
-               r = runtime·fastrand1();
-       return r%n;
-}