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.w...@oracle.com> Signed-off-by: Keith Busch <keith.bu...@intel.com> --- 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