Fix cv_timedwait timeout

Perform the already past expiration time check before updating
cvp->cv_mutex with the provided mutex.  This check only depends
on local state.  Doing it first ensures that cvp->cv_mutex will not
be updated in the timeout case or if it's ever called with an
expire_time <= now.

Reviewed-by: Tim Chase <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes openzfs#616

module/spl/spl-condvar.c

@@ -166,22 +166,19 @@ __cv_timedwait_common(kcondvar_t *cvp, kmutex_t *mp, clock_t expire_time,
 	ASSERT(mp);
 	ASSERT(cvp->cv_magic == CV_MAGIC);
 	ASSERT(mutex_owned(mp));
-	atomic_inc(&cvp->cv_refs);
 
+	/* XXX - Does not handle jiffie wrap properly */
+	time_left = expire_time - jiffies;
+	if (time_left <= 0)
+		return (-1);
+
+	atomic_inc(&cvp->cv_refs);
 	m = ACCESS_ONCE(cvp->cv_mutex);
 	if (!m)
 		m = xchg(&cvp->cv_mutex, mp);
 	/* Ensure the same mutex is used by all callers */
 	ASSERT(m == NULL || m == mp);
 
-	/* XXX - Does not handle jiffie wrap properly */
-	time_left = expire_time - jiffies;
-	if (time_left <= 0) {
-		/* XXX - doesn't reset cv_mutex */
-		atomic_dec(&cvp->cv_refs);
-		return (-1);
-	}
-
 	prepare_to_wait_exclusive(&cvp->cv_event, &wait, state);
 	atomic_inc(&cvp->cv_waiters);
 
@@ -238,28 +235,25 @@ __cv_timedwait_hires(kcondvar_t *cvp, kmutex_t *mp, hrtime_t expire_time,
 {
 	DEFINE_WAIT(wait);
 	kmutex_t *m;
-	hrtime_t time_left, now;
+	hrtime_t time_left;
 	ktime_t ktime_left;
 
 	ASSERT(cvp);
 	ASSERT(mp);
 	ASSERT(cvp->cv_magic == CV_MAGIC);
 	ASSERT(mutex_owned(mp));
-	atomic_inc(&cvp->cv_refs);
 
+	time_left = expire_time - gethrtime();
+	if (time_left <= 0)
+		return (-1);
+
+	atomic_inc(&cvp->cv_refs);
 	m = ACCESS_ONCE(cvp->cv_mutex);
 	if (!m)
 		m = xchg(&cvp->cv_mutex, mp);
 	/* Ensure the same mutex is used by all callers */
 	ASSERT(m == NULL || m == mp);
 
-	now = gethrtime();
-	time_left = expire_time - now;
-	if (time_left <= 0) {
-		atomic_dec(&cvp->cv_refs);
-		return (-1);
-	}
-
 	prepare_to_wait_exclusive(&cvp->cv_event, &wait, state);
 	atomic_inc(&cvp->cv_waiters);