# HG changeset patch
# User Erik Quanstrom <quanstro@quanstro.net>
# Date 1329518202 0
# Node ID d5c3a985ece44b36bd0bb86aeae6c0400e1666f4
# Parent  cac66f146601bb39dfe9ec8f14f16cb02375f3a1
tsemacquire: uncoherent access to ticks

cavet: i don't have any code for testing this.
i noticed it by accident.

i see two problems with the original code,
1.  sleep can pop out on a different processor
than it entered on, so i believe sys->ticks is
correct.  otherwise we require that ∀i, j in
machnos, m(i)->ticks ≡ m(j)->ticks.

2. X->ticks - t is comuted twice, but it is
not guarenteed to produce the same value
each time.

R=nixiedev, john, seed
CC=nix-dev
http://codereview.appspot.com/5677051

Committer: John Floren <john@jfloren.net>

diff -r cac66f146601 -r d5c3a985ece4 sys/src/nix/port/sysproc.c
--- a/sys/src/nix/port/sysproc.c	Sat Feb 18 13:49:14 2012 +0000
+++ b/sys/src/nix/port/sysproc.c	Fri Feb 17 22:36:42 2012 +0000
@@ -1182,12 +1182,11 @@
 
 /* Acquire semaphore or time-out */
 static int
-tsemacquire(Segment* s, int* addr, ulong ms)
+tsemacquire(Segment* s, int* addr, long ms)
 {
 	int acquired;
+	ulong t;
 	Sema phore;
-	int timedout;
-	ulong t;
 
 	if(canacquire(addr))
 		return 1;
@@ -1195,7 +1194,6 @@
 		return 0;
 
 	acquired = 0;
-	timedout = 0;
 	semqueue(s, addr, &phore);
 	for(;;){
 		phore.waiting = 1;
@@ -1206,21 +1204,18 @@
 		}
 		if(waserror())
 			break;
-		t = m->ticks;
+		t = sys->ticks;
 		tsleep(&phore, semawoke, &phore, ms);
-		if(TK2MS(m->ticks-t) >= ms) {
-			timedout = 1;
-			poperror();
+		ms -= TK2MS(sys->ticks-t);
+		poperror();
+		if(ms <= 0)
 			break;
-		}
-		ms -= TK2MS(m->ticks-t);
-		poperror();
 	}
 	semdequeue(s, &phore);
 	coherence();	/* not strictly necessary due to lock in semdequeue */
 	if(!phore.waiting)
 		semwakeup(s, addr, 1);
-	if(timedout)
+	if(ms <= 0)
 		return 0;
 	if(!acquired)
 		nexterror();