[commit: ghc] master: Fix bug causing polling instead of blocking in the non-threaded RTS (#5773) (fac8ecb)

[Simon Marlow]

Repository : ssh://darcs.haskell.org//srv/darcs/ghc

On branch  : master

http://hackage.haskell.org/trac/ghc/changeset/fac8ecbbafde17dd92439c41747223c43e9d2b80

>---------------------------------------------------------------

commit fac8ecbbafde17dd92439c41747223c43e9d2b80
Author: Simon Marlow <marlowsd at gmail.com>
Date:   Mon Jan 16 09:49:24 2012 +0000

    Fix bug causing polling instead of blocking in the non-threaded RTS (#5773)
    
    This was a regression introduced accidentally in
    6b1098511aaabd2c9503ee7be6da1944466f9cb4.  We were previously passing
    a large time value to select() to simulate blocking, and this broke
    due to a change from unsigned to signed arithmetic.  I've refactored
    it to be less fragile now - we just pass NULL as the timeval parameter
    to select(), which is the correct way to do blocking.

>---------------------------------------------------------------

 rts/posix/Select.c |   33 ++++++++++++++++-----------------
 1 files changed, 16 insertions(+), 17 deletions(-)

diff --git a/rts/posix/Select.c b/rts/posix/Select.c
index 45737ce..ffe72e8 100644
--- a/rts/posix/Select.c
+++ b/rts/posix/Select.c
@@ -118,13 +118,9 @@ awaitEvent(rtsBool wait)
     int maxfd = -1;
     rtsBool select_succeeded = rtsTrue;
     rtsBool unblock_all = rtsFalse;
-    struct timeval tv;
-    Time min;
+    struct timeval tv, *ptv;
     LowResTime now;
 
-    tv.tv_sec  = 0;
-    tv.tv_usec = 0;
-    
     IF_DEBUG(scheduler,
 	     debugBelch("scheduler: checking for threads blocked on I/O");
 	     if (wait) {
@@ -145,14 +141,6 @@ awaitEvent(rtsBool wait)
 	  return;
       }
 
-      if (!wait) {
-	  min = 0;
-      } else if (sleeping_queue != END_TSO_QUEUE) {
-          min = LowResTimeToTime(sleeping_queue->block_info.target - now);
-      } else {
-          min = (Time)-1;
-      }
-
       /* 
        * Collect all of the fd's that we're interested in
        */
@@ -194,12 +182,23 @@ awaitEvent(rtsBool wait)
 	}
       }
 
-      /* Check for any interesting events */
+      if (wait) {
+          ptv = NULL;
+      } else if (sleeping_queue != END_TSO_QUEUE) {
+          Time min = LowResTimeToTime(sleeping_queue->block_info.target - now);
+          tv.tv_sec  = TimeToSeconds(min);
+          tv.tv_usec = TimeToUS(min) % 1000000;
+          ptv = &tv;
+      } else {
+          // just poll
+          tv.tv_sec  = 0;
+          tv.tv_usec = 0;
+          ptv = &tv;
+      }
       
-      tv.tv_sec  = TimeToSeconds(min);
-      tv.tv_usec = TimeToUS(min) % 1000000;
+      /* Check for any interesting events */
 
-      while ((numFound = select(maxfd+1, &rfd, &wfd, NULL, &tv)) < 0) {
+      while ((numFound = select(maxfd+1, &rfd, &wfd, NULL, ptv)) < 0) {
 	  if (errno != EINTR) {
 	    /* Handle bad file descriptors by unblocking all the
 	       waiting threads. Why? Because a thread might have been