[PATCH v4 1/6] blk-mq: Make it safe to use RCU to iterate over blk_mq_tag_set.tag_list

[Bart Van Assche]

Since the next patch in this series will use RCU to iterate over
tag_list, make this safe. Add lockdep_assert_held() statements
in functions that iterate over tag_list to make clear that using
list_for_each_entry() instead of list_for_each_entry_rcu() is
fine in these functions.

Signed-off-by: Bart Van Assche <bart.vanass...@sandisk.com>
Cc: Christoph Hellwig <h...@lst.de>
Cc: Hannes Reinecke <h...@suse.com>
---
 block/blk-mq.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index f7cd3208bcdf..c26464f9649a 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2076,6 +2076,8 @@ static void blk_mq_update_tag_set_depth(struct 
blk_mq_tag_set *set, bool shared)
 {
        struct request_queue *q;
 
+       lockdep_assert_held(&set->tag_list_lock);
+
        list_for_each_entry(q, &set->tag_list, tag_set_list) {
                blk_mq_freeze_queue(q);
                queue_set_hctx_shared(q, shared);
@@ -2088,7 +2090,8 @@ static void blk_mq_del_queue_tag_set(struct request_queue 
*q)
        struct blk_mq_tag_set *set = q->tag_set;
 
        mutex_lock(&set->tag_list_lock);
-       list_del_init(&q->tag_set_list);
+       list_del_rcu(&q->tag_set_list);
+       INIT_LIST_HEAD(&q->tag_set_list);
        if (list_is_singular(&set->tag_list)) {
                /* just transitioned to unshared */
                set->flags &= ~BLK_MQ_F_TAG_SHARED;
@@ -2096,6 +2099,8 @@ static void blk_mq_del_queue_tag_set(struct request_queue 
*q)
                blk_mq_update_tag_set_depth(set, false);
        }
        mutex_unlock(&set->tag_list_lock);
+
+       synchronize_rcu();
 }
 
 static void blk_mq_add_queue_tag_set(struct blk_mq_tag_set *set,
@@ -2113,7 +2118,7 @@ static void blk_mq_add_queue_tag_set(struct 
blk_mq_tag_set *set,
        }
        if (set->flags & BLK_MQ_F_TAG_SHARED)
                queue_set_hctx_shared(q, true);
-       list_add_tail(&q->tag_set_list, &set->tag_list);
+       list_add_tail_rcu(&q->tag_set_list, &set->tag_list);
 
        mutex_unlock(&set->tag_list_lock);
 }
@@ -2601,6 +2606,8 @@ void blk_mq_update_nr_hw_queues(struct blk_mq_tag_set 
*set, int nr_hw_queues)
 {
        struct request_queue *q;
 
+       lockdep_assert_held(&set->tag_list_lock);
+
        if (nr_hw_queues > nr_cpu_ids)
                nr_hw_queues = nr_cpu_ids;
        if (nr_hw_queues < 1 || nr_hw_queues == set->nr_hw_queues)
-- 
2.12.0