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

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PR 850/rtems
	* 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.
This commit is contained in:
Joel Sherrill
2006-03-07 22:09:49 +00:00
parent 5b07faf305
commit ede4f1621a
2 changed files with 44 additions and 3 deletions
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13
cpukit/score/ChangeLog
View File

@@ -1,3 +1,16 @@
2006-03-07 Steven Johnson <sjohnson@sakuraindustries.com>
PR 850/rtems
* 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.
2006-03-07 Joel Sherrill <joel@OARcorp.com> 2006-03-07 Joel Sherrill <joel@OARcorp.com>
PR 866/rtems PR 866/rtems

34
cpukit/score/src/watchdogtickle.c
View File

@@ -49,9 +49,37 @@ void _Watchdog_Tickle(
goto leave; goto leave;
the_watchdog = _Watchdog_First( header ); 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 { do {
watchdog_state = _Watchdog_Remove( the_watchdog ); watchdog_state = _Watchdog_Remove( the_watchdog );