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

[Christoph Hellwig]

Looks good,

Reviewed-by: Christoph Hellwig 

[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 
Cc: Christoph Hellwig 
Cc: Hannes Reinecke 
---
 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(>tag_list_lock);
+
list_for_each_entry(q, >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(>tag_list_lock);
-   list_del_init(>tag_set_list);
+   list_del_rcu(>tag_set_list);
+   INIT_LIST_HEAD(>tag_set_list);
if (list_is_singular(>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(>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(>tag_set_list, >tag_list);
+   list_add_tail_rcu(>tag_set_list, >tag_list);
 
mutex_unlock(>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(>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