A recent commit had tag iterator callbacks run under the rcu read
lock. Existing callbacks exist that do not satisy the rcu non-blocking
requirement.
The commit intended to prevent an iterator from accessing a queue that's
being modified. This patch fixes the original issue by taking a queue
reference if the queue is not frozen instead of a rcu read lock.
Fixes: f5bbbbe4d6357 ("blk-mq: sync the update nr_hw_queues with blk_mq_queue_tag_busy_iter")
Cc: Jianchao Wang <jianchao.w.wang@xxxxxxxxxx>
Signed-off-by: Keith Busch <keith.busch@xxxxxxxxx>
---
block/blk-mq-tag.c | 30 +++++++++++++++++-------------
block/blk-mq-tag.h | 2 +-
block/blk-mq.c | 22 +---------------------
3 files changed, 19 insertions(+), 35 deletions(-)
diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 94e1ed667b6e..63542642a017 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -314,24 +314,27 @@ void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
}
EXPORT_SYMBOL(blk_mq_tagset_busy_iter);
-void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
+bool blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
void *priv)
{
struct blk_mq_hw_ctx *hctx;
int i;
- /*
- * __blk_mq_update_nr_hw_queues will update the nr_hw_queues and
- * queue_hw_ctx after freeze the queue. So we could use q_usage_counter
- * to avoid race with it. __blk_mq_update_nr_hw_queues will users
- * synchronize_rcu to ensure all of the users go out of the critical
- * section below and see zeroed q_usage_counter.
+ /* A deadlock might occur if a request is stuck requiring a
+ * timeout at the same time a queue freeze is waiting
+ * completion, since the timeout code would not be able to
+ * acquire the queue reference here.
+ *
+ * That's why we don't use blk_queue_enter here; instead, we use
+ * percpu_ref_tryget directly, because we need to be able to
+ * obtain a reference even in the short window between the queue
+ * starting to freeze, by dropping the first reference in
+ * blk_freeze_queue_start, and the moment the last request is
+ * consumed, marked by the instant q_usage_counter reaches
+ * zero.
*/
- rcu_read_lock();
- if (percpu_ref_is_zero(&q->q_usage_counter)) {
- rcu_read_unlock();
- return;
- }
+ if (!percpu_ref_tryget(&q->q_usage_counter))
+ return false;
queue_for_each_hw_ctx(q, hctx, i) {
struct blk_mq_tags *tags = hctx->tags;
@@ -347,7 +350,8 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
bt_for_each(hctx, &tags->breserved_tags, fn, priv, true);
bt_for_each(hctx, &tags->bitmap_tags, fn, priv, false);
}
- rcu_read_unlock();
+ blk_queue_exit(q);
+ return true;
}
static int bt_alloc(struct sbitmap_queue *bt, unsigned int depth,
diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h
index 61deab0b5a5a..36b3bc90e867 100644
--- a/block/blk-mq-tag.h
+++ b/block/blk-mq-tag.h
@@ -33,7 +33,7 @@ extern int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
struct blk_mq_tags **tags,
unsigned int depth, bool can_grow);
extern void blk_mq_tag_wakeup_all(struct blk_mq_tags *tags, bool);
-void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
+bool blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
void *priv);
static inline struct sbq_wait_state *bt_wait_ptr(struct sbitmap_queue *bt,
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 85a1c1a59c72..019f9b169887 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -848,24 +848,9 @@ static void blk_mq_timeout_work(struct work_struct *work)
struct blk_mq_hw_ctx *hctx;
int i;
- /* A deadlock might occur if a request is stuck requiring a
- * timeout at the same time a queue freeze is waiting
- * completion, since the timeout code would not be able to
- * acquire the queue reference here.
- *
- * That's why we don't use blk_queue_enter here; instead, we use
- * percpu_ref_tryget directly, because we need to be able to
- * obtain a reference even in the short window between the queue
- * starting to freeze, by dropping the first reference in
- * blk_freeze_queue_start, and the moment the last request is
- * consumed, marked by the instant q_usage_counter reaches
- * zero.
- */
- if (!percpu_ref_tryget(&q->q_usage_counter))
+ if (!blk_mq_queue_tag_busy_iter(q, blk_mq_check_expired, &next))
return;
- blk_mq_queue_tag_busy_iter(q, blk_mq_check_expired, &next);
-
if (next != 0) {
mod_timer(&q->timeout, next);
} else {
@@ -881,7 +866,6 @@ static void blk_mq_timeout_work(struct work_struct *work)
blk_mq_tag_idle(hctx);
}
}
- blk_queue_exit(q);
}
struct flush_busy_ctx_data {
@@ -2974,10 +2958,6 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
list_for_each_entry(q, &set->tag_list, tag_set_list)
blk_mq_freeze_queue(q);
- /*
- * Sync with blk_mq_queue_tag_busy_iter.
- */
- synchronize_rcu();
/*
* Switch IO scheduler to 'none', cleaning up the data associated
* with the previous scheduler. We will switch back once we are done
--
2.14.4
