It was possible to set an alarm to abort a syscall and break out of the
alarm handler while blocked on an abortable syscall.
Signed-off-by: Barret Rhoden <b...@cs.berkeley.edu>
---
user/parlib/alarm.c | 23 ++++++++++++++---------
1 file changed, 14 insertions(+), 9 deletions(-)
diff --git a/user/parlib/alarm.c b/user/parlib/alarm.c
index 0e1b336f9e89..a51841422a71 100644
--- a/user/parlib/alarm.c
+++ b/user/parlib/alarm.c
@@ -472,15 +472,20 @@ void print_chain(struct timer_chain *tchain)
void alarm_abort_sysc(struct alarm_waiter *awaiter)
{
struct uthread *uth = awaiter->data;
+
assert(uth);
- if (!uth->sysc) {
- /* It's possible the sysc hasn't blocked yet or is in the
process of
- * unblocking, or even has returned, but hasn't cancelled the
alarm.
- * regardless, we request a new alarm (the uthread will cancel
us one
- * way or another). */
- set_awaiter_rel(awaiter, 10000);
- __set_alarm(awaiter);
+ if (uth->sysc && sys_abort_sysc(uth->sysc))
return;
- }
- sys_abort_sysc(uth->sysc);
+ /* There are a bunch of reasons why we didn't abort the syscall. The
+ * syscall might not have been issued or blocked at all, so uth->sysc
would
+ * be NULL. The syscall might have blocked, but at a non-abortable
location
+ * - picture blocking on a qlock, then unblocking and blocking later on
a
+ * rendez. If you try to abort in between, abort_sysc will fail, then
we'll
+ * get blocked on the rendez until the next abort. Finally, the syscall
+ * might have completed, but the uthread hasn't cancelled the alarm yet.
+ *
+ * It's always safe to rearm the alarm - the uthread will unset it and
break
+ * us out of the rearm loop. */
+ set_awaiter_rel(awaiter, 10000);
+ __set_alarm(awaiter);
}
--
2.18.0.865.gffc8e1a3cd6-goog
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