blk-iocost: fix incorrect vtime comparison in iocg_is_idle()

vtimes may wrap and time_before/after64() should be used to determine
whether a given vtime is before or after another. iocg_is_idle() was
incorrectly using plain "<" comparison do determine whether done_vtime
is before vtime. Here, the only thing we're interested in is whether
done_vtime matches vtime which indicates that there's nothing in
flight. Let's test for inequality instead.

Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 7caa471 ("blkcg: implement blk-iocost")
Cc: stable@vger.kernel.org # v5.4+
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-iocost.c

@@ -1318,7 +1318,7 @@ static bool iocg_is_idle(struct ioc_gq *iocg)
 		return false;
 
 	/* is something in flight? */
-	if (atomic64_read(&iocg->done_vtime) < atomic64_read(&iocg->vtime))
+	if (atomic64_read(&iocg->done_vtime) != atomic64_read(&iocg->vtime))
 		return false;
 
 	return true;