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kernel: tsemacquire() use MACHP(0)->ticks for time delta

we might wake up on a different cpu after the sleep so
delta from machX->ticks - machY->ticks can become negative
giving spurious timeouts. to avoid this always use the
same mach 0 tick counter for the delta.

--- a/sys/src/9/port/sysproc.c

+++ b/sys/src/9/port/sysproc.c

@@ -1126,7 +1126,7 @@

 tsemacquire(Segment *s, long *addr, ulong ms)

 {

 	int acquired, timedout;

-	ulong t, elms;

+	ulong t;

 	Sema phore;

 	if(canacquire(addr))

@@ -1144,15 +1144,15 @@

 		}

 		if(waserror())

 			break;

-		t = m->ticks;

+		t = MACHP(0)->ticks;

 		tsleep(&phore, semawoke, &phore, ms);

-		elms = TK2MS(m->ticks - t);

+		t = TK2MS(MACHP(0)->ticks - t);

 		poperror();

-		if(elms >= ms){

+		if(t >= ms){

 			timedout = 1;

 			break;

 		}

-		ms -= elms;

+		ms -= t;

 	}

 	semdequeue(s, &phore);

 	coherence();	/* not strictly necessary due to lock in semdequeue */