- /* Avoid obd_uncommitted_replies_lock contention if we 100% sure that
- * rs has been removed from the list already */
- if (!list_empty_careful(&rs->rs_obd_list)) {
- spin_lock(&obd->obd_uncommitted_replies_lock);
+ /* The disk commit callback holds exp_uncommitted_replies_lock while it
+ * iterates over newly committed replies, removing them from
+ * exp_uncommitted_replies. It then drops this lock and schedules the
+ * replies it found for handling here.
+ *
+ * We can avoid contention for exp_uncommitted_replies_lock between the
+ * HRT threads and further commit callbacks by checking rs_committed
+ * which is set in the commit callback while it holds both
+ * rs_lock and exp_uncommitted_reples.
+ *
+ * If we see rs_committed clear, the commit callback _may_ not have
+ * handled this reply yet and we race with it to grab
+ * exp_uncommitted_replies_lock before removing the reply from
+ * exp_uncommitted_replies. Note that if we lose the race and the
+ * reply has already been removed, list_del_init() is a noop.
+ *
+ * If we see rs_committed set, we know the commit callback is handling,
+ * or has handled this reply since store reordering might allow us to
+ * see rs_committed set out of sequence. But since this is done
+ * holding rs_lock, we can be sure it has all completed once we hold
+ * rs_lock, which we do right next.
+ */
+ if (!rs->rs_committed) {
+ spin_lock(&exp->exp_uncommitted_replies_lock);