[PATCH] sim: fix signed 32bit time overflow with long delayed events

Mike Frysinger
2010-04-20T11:16:06Z

The sim-events code jumps through some hoops to avoid using 64bit math
to manage the current time. One fundamental assumption here is that by
constantly scheduling the sim poll event a short time into the future,
the 64bit difference will always fall into a signed 32bit value. This
does work most of the time, except for when processing the sim poll event
itself.

Normally, sim_events_process() will dequeue the sim poll event, update
the current time (time_from_event) according to the next pending event,
process the sim poll event (which will then requeue the sim poll event),
and then continue on.

The problem here of course is that the current time is updated in that
small window before the sim poll event gets a chance to reschedule itself.
So if the 64bit difference between the current time and the next event
does not fit into the signed 32bit value, time_from_event overflows, and
the internal assert at the end of update_time_from_event() triggers.

This was noticed when simulating Blackfin Das U-Boot. The simulated core
timer is given the max unsigned timeout value possible on a 32bit processor
(0xffffffff). This timeout value is used directly to schedule a hw event
in the sim future (the IRQ firing). Once the sim poll event is kicked off,
the next pending event is the core timer event which is more than 2^31
ticks in the future, and the sim aborts with:
sim-events.c:435: assertion failed - current_time == sim_events_time (sd)

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
---
2010-04-20 Mike Frysinger <vapier@gentoo.org>

* sim-events.c (sim_events_process): Delay update_time_from_event()
till after the call to handler().

sim/common/sim-events.c | 5 ++++-
1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/sim/common/sim-events.c b/sim/common/sim-events.c
index e02fb76..3fa5433 100644
--- a/sim/common/sim-events.c
+++ b/sim/common/sim-events.c
@@ -1172,7 +1172,6 @@ sim_events_process (SIM_DESC sd)
sim_event_handler *handler = to_do->handler;
void *data = to_do->data;
events->queue = to_do->next;
- update_time_from_event (sd);
ETRACE((_ETRACE,
"event issued at %ld - tag 0x%lx - handler 0x%lx, data 0x%lx%s%s\n",
(long) event_time,
@@ -1183,6 +1182,10 @@ sim_events_process (SIM_DESC sd)
(to_do->trace != NULL) ? to_do->trace : ""));
sim_events_free (sd, to_do);
handler (sd, data);
+ /* Update time *after* the handler in case it inserted an event itself.
+ Like in the case of the persistent sim poll event. Otherwise, our
+ 32bit math assumption may be violated and overflow. */
+ update_time_from_event (sd);
}

/* put things back where they belong ready for the next iteration */
--
1.7.0.2