runtime: wait to allocate mach semaphores backing Locks until needed

need better management of mach semaphores eventually
but this avoids allocating them for uncontended Locks.

R=r
CC=agl1, golang-dev
https://golang.org/cl/190079

diff --git a/src/pkg/runtime/darwin/thread.c b/src/pkg/runtime/darwin/thread.c
index f68b63f7ab89f728e76153a53c6980d810c77bd6..2f0bee9e6f59f0fbbad01f093cd74106bf198493 100644 (file)
--- a/src/pkg/runtime/darwin/thread.c
+++ b/src/pkg/runtime/darwin/thread.c
@@ -60,12 +60,12 @@ lock(Lock *l)
                throw("lock count");
        m->locks++;
 
-       // Allocate semaphore if needed.
-       if(l->sema == 0)
-               initsema(&l->sema);
-
-       if(xadd(&l->key, 1) > 1)        // someone else has it; wait
+       if(xadd(&l->key, 1) > 1) {      // someone else has it; wait
+               // Allocate semaphore if needed.
+               if(l->sema == 0)
+                       initsema(&l->sema);
                mach_semacquire(l->sema);
+       }
 }
 
 void
@@ -75,8 +75,12 @@ unlock(Lock *l)
        if(m->locks < 0)
                throw("lock count");
 
-       if(xadd(&l->key, -1) > 0)       // someone else is waiting
+       if(xadd(&l->key, -1) > 0) {     // someone else is waiting
+               // Allocate semaphore if needed.
+               if(l->sema == 0)
+                       initsema(&l->sema);
                mach_semrelease(l->sema);
+       }
 }
 
 
@@ -87,15 +91,21 @@ unlock(Lock *l)
 void
 usemacquire(Usema *s)
 {
-       if((int32)xadd(&s->u, -1) < 0)
+       if((int32)xadd(&s->u, -1) < 0) {
+               if(s->k == 0)
+                       initsema(&s->k);
                mach_semacquire(s->k);
+       }
 }
 
 void
 usemrelease(Usema *s)
 {
-       if((int32)xadd(&s->u, 1) <= 0)
+       if((int32)xadd(&s->u, 1) <= 0) {
+               if(s->k == 0)
+                       initsema(&s->k);
                mach_semrelease(s->k);
+       }
 }
 
 
@@ -109,8 +119,6 @@ noteclear(Note *n)
 void
 notesleep(Note *n)
 {
-       if(n->sema.k == 0)
-               initsema(&n->sema.k);
        while(!n->wakeup)
                usemacquire(&n->sema);
 }
@@ -118,8 +126,6 @@ notesleep(Note *n)
 void
 notewakeup(Note *n)
 {
-       if(n->sema.k == 0)
-               initsema(&n->sema.k);
        n->wakeup = 1;
        usemrelease(&n->sema);
 }