2006-03-07 Steven Johnson <sjohnson@sakuraindustries.com>

	PR 850/rtems
	* score/src/watchdogtickle.c: A Watchdog (used to timeout an event)
	with a delay of 1 sometimes does not seem to timeout.  The problem
	occurs, because for whatever reason when the watchdog tickle function
	executes, the watchdog->delta_interval is 0. it is then decremented
	before being tested, becomes huge and so doesnt time out.  It is
	thought there is a race condition where the watchdog->delta_interval
	is calculated by reference to a head (also with a delay of 1). But
	before it can be added after the head, the head is removed, so the
	new head now has a delay of 0.

1 files changed, 31 insertions, 3 deletions

diff --git a/cpukit/score/src/watchdogtickle.c b/cpukit/score/src/watchdogtickle.c
index 82b614894b..2fca11b8ac 100644
--- a/cpukit/score/src/watchdogtickle.c
+++ b/cpukit/score/src/watchdogtickle.c
@@ -53,9 +53,37 @@ void _Watchdog_Tickle(
     goto leave;
 
   the_watchdog = _Watchdog_First( header );
-  the_watchdog->delta_interval--;
-  if ( the_watchdog->delta_interval != 0 )
-    goto leave;
+  
+  /*
+   * For some reason, on rare occasions the_watchdog->delta_interval
+   * of the head of the watchdog chain is 0.  Before this test was
+   * added, on these occasions an event (which usually was supposed
+   * to have a timeout of 1 tick would have a delta_interval of 0, which
+   * would be decremented to 0xFFFFFFFF by the unprotected 
+   * "the_watchdog->delta_interval--;" operation.
+   * This would mean the event would not timeout, and also the chain would
+   * be blocked, because a timeout with a very high number would be at the
+   * head, rather than at the end.
+   * The test "if (the_watchdog->delta_interval != 0)"
+   * here prevents this from occuring. 
+   * 
+   * We were not able to categorically identify the situation that causes 
+   * this, but proved it to be true empirically.  So this check causes 
+   * correct behaviour in this circumstance.
+   * 
+   * The belief is that a race condition exists whereby an event at the head
+   * of the chain is removed (by a pending ISR or higher priority task)
+   * during the _ISR_Flash( level ); in _Watchdog_Insert, but the watchdog 
+   * to be inserted has already had its delta_interval adjusted to 0, and 
+   * so is added to the head of the chain with a delta_interval of 0.  
+   * 
+   * Steven Johnson - 12/2005 (gcc-3.2.3 -O3 on powerpc)
+   */
+  if (the_watchdog->delta_interval != 0) {
+    the_watchdog->delta_interval--;
+    if ( the_watchdog->delta_interval != 0 )
+      goto leave;
+  }      
 
   do {
 	 watchdog_state = _Watchdog_Remove( the_watchdog );