From: Kirk McKusick <mckusick@ucbvax.Berkeley.EDU>
Date: Thu, 11 Aug 1994 13:19:13 +0000 (-0800)
Subject: a zero-valued timeout (which should poll and return) gets turned into
X-Git-Url: https://git.subgeniuskitty.com/unix-history/.git/commitdiff_plain/630c291e5b4070bf7e5a7894b82e9699961b61d1

a zero-valued timeout (which should poll and return) gets turned into
a one-tick timeout, and (worse) the amount of time to sleep is not
recomputed after each sleep.  I.e., if the timeout is 10 sec, and
select is incorrectly after 9 sec, it will go back to sleep for up
to 10 sec.  In fact, in the worst case it will never return. (from karels)

SCCS-vsn: sys/kern/sys_generic.c 8.7
---

diff --git a/usr/src/sys/kern/sys_generic.c b/usr/src/sys/kern/sys_generic.c
index fc852d5b15..be609f20f7 100644
--- a/usr/src/sys/kern/sys_generic.c
+++ b/usr/src/sys/kern/sys_generic.c
@@ -9,7 +9,7 @@
  *
  * %sccs.include.redist.c%
  *
- *	@(#)sys_generic.c	8.6 (Berkeley) %G%
+ *	@(#)sys_generic.c	8.7 (Berkeley) %G%
  */
 
 #include <sys/param.h>
@@ -470,7 +470,7 @@ select(p, uap, retval)
 {
 	fd_set ibits[3], obits[3];
 	struct timeval atv;
-	int s, ncoll, error = 0, timo;
+	int s, ncoll, error = 0, timo, doblock;
 	u_int ni;
 
 	bzero((caddr_t)ibits, sizeof(ibits));
@@ -499,14 +499,14 @@ select(p, uap, retval)
 			error = EINVAL;
 			goto done;
 		}
-		s = splclock();
-		timevaladd(&atv, (struct timeval *)&time);
-		timo = hzto(&atv);
 		/*
-		 * Avoid inadvertently sleeping forever.
+		 * Don't let a short time get rounded down to zero
+		 * and cause us to sleep forever, but exactly zero
+		 * means "do not block".
 		 */
-		if (timo == 0)
-			timo = 1;
+		doblock = (atv.tv_usec || atv.tv_sec);
+		s = splclock();
+		timevaladd(&atv, (struct timeval *)&time);
 		splx(s);
 	} else
 		timo = 0;
@@ -517,17 +517,24 @@ retry:
 	if (error || *retval)
 		goto done;
 	s = splhigh();
-	/* this should be timercmp(&time, &atv, >=) */
-	if (uap->tv && (time.tv_sec > atv.tv_sec ||
-	    time.tv_sec == atv.tv_sec && time.tv_usec >= atv.tv_usec)) {
-		splx(s);
-		goto done;
+	if (uap->tv) {
+		if (timercmp(&time, &atv, >=)) {
+			splx(s);
+			goto done;
+		}
+		timo = hzto(&atv);
+		/*
+		 * Avoid inadvertently sleeping forever.
+		 */
+		if (doblock && timo == 0)
+			timo = 1;
 	}
 	if ((p->p_flag & P_SELECT) == 0 || nselcoll != ncoll) {
 		splx(s);
 		goto retry;
 	}
 	p->p_flag &= ~P_SELECT;
+	doblock = 0;
 	error = tsleep((caddr_t)&selwait, PSOCK | PCATCH, "select", timo);
 	splx(s);
 	if (error == 0)